Polish for quick-access pie menus

* Settings pie menu (DQ) can now be used to quickly check on and adjust
key properties for the active layer. In particular, it's particularly
helpful for quickly checking colours and turning onion skinning on/off.

* Fleshed out the "More" pie with many of the other tools as well. Also
included a link back to the main tools pie (i.e. the one launched with Ctrl-D)

Experimenting with contents of secondary GPencil pie

Now that we can properly add interactive widgets, this commit adds some
quick access to some commonly tweaked settings such as color (stroke/fill),
opacity, line thickness, and onionskinning.

Merge branch 'master' into GPencil_EditStrokes

Standard hotkeys to set proportional editing settings work in Stroke EditMode too

WIP convenience feature: New frames are created when editing strokes on frames without existing keys

This feature makes it easier to do pose-to-pose animation using the strokes editing
stuff, as you don't have to remember to deliberately duplicate an existing frame
so that your edits end up on a new frame.

There is the possibility though that you didn't intend to create a new frame,
in which case this option will be more of a pain. Let's try this out for a bit
before deciding whether it should stay or go.

Notifier fixes for entering/exiting stroke editmode (and other fixes)

* Entering/exiting stroke editmode now updates the headers, so that the right
proportional editing options get shown

* Doing anything Grease Pencil related in the Node Editor doesn't trigger a
recalc of the nodetree anymore. AFAICT, there is no legitimate reason to
be doing full refreshes there - simple redraws will do.

Add proportional editing props to the "additional settings" panel where they might be relevant

Proportional Editing options for GPencil should now work across all editors supported

* When in Stroke Editing mode (or strokes points can get transformed),
the proportional editing settings for editing points/geometry will now
be used. These include the disabled/enabled/connected + falloff properties.

* Uncommented the code in the pie menu for showing these settings, so that
this functionality can be tested in each applicable editor.

TODO:
* Fix refresh bugs for UI to ensure that the right set of settings is displayed
* Expose these settings in other places where they may be applicable

GPencil: Proportional Editing UI components for headers

This is only the UI part - the backend doesn't work yet

Bugfix: Undo during "continous drawing" (or "sketching sessions") for GPencil was broken

While the hotkey here is now hardcoded, it is possible to undo the last
stroke when drawing with the "continous drawing" option enabled. This
seems to have been broken at some stage due to a fix the old node-editor
bug where holding the dkey was accidentally inserting drivers all over
the show.

This bug seems to have been around since 2.69 (at least!), and doesn't seem
to have been noticed until now!

Quick hacky fix for crash on undo (for Grease Pencil)

AnimData is not copied when making duplicating bGPdata for internal usage
(i.e. for the undo buffer); instead, the data was being shared between the
real copies and the undo buffer copies (to save memory - since otherwise
we'd have to have copies of all the animation data floating around).

The fix here clears these refs before trying to free these copies,
making sure that the data won't get freed incorrectly.

Another attempt at making the GPencil pie menu more useful

Selection operators are now back in the main menu, along with tools for
duplicating and deleting. For now, mirror and bend are still in their own
separate pie that can be accessed via the "More..." entry.

Fix: Doing OpenGL renders of GPencil sketches drawn in sequencer won't show points now

Tweaks to GPencil keymaps

* Moved pie menu to Ctrl-D instead. DD turned out to be far too easy to accidentally
trigger when quickly trying to draw 2 strokes in rapid succession.
* Disabled for now the hotkeys for 'straight line' and 'poly' draw tools. These
weren't that easy to remember/use, and were getting blocked by the new pie menu
anyway.

Merge branch 'master' into GPencil_EditStrokes

Conflicts:
source/blender/editors/gpencil/drawgpencil.c
source/blender/editors/gpencil/gpencil_ops.c

Animation Editor support for animating Grease Pencil settings

Experiment: Some tweaks to the Grease Pencil pie menu layout

Split off selection tools and copy (+ a few others not previously included)
into secondary "Select..." and "More..." pie menus accessible from the first.
This helps balance out the menu more in order to have consistency between
edit and non-editmode (i.e. the toggle option is now always in the same place),
as well as opening up more space for new tools to become accessible.

It does however seem a lot clunkier than the previous approach...

Reshuffling of extra tool settings

Previously it was necessary to expose the editmode and stroke placement settings
at the bottom of the settings panel when it was displayed in the Sequencer, as
the sequencer doesn't have a toolbar for hosting those settings.

This commit moves those out into a separate panel, which can be included in other
editors too. This applies to editors which have a toolshelf for displaying these
settings too (e.g. nodes editor), where the toolshelf is often collapsed, making it
valuable to be able to set these settings from more than one place.

(While not great from a long-term standpoint, such duplication is a "good enough"
solution in the meantime, until we figure out a better solution)

Merge branch 'master' into GPencil_EditStrokes

Remove check for "Grease Pencil Mode" from color swatch drawing for GP Layer entries
in preparation for upcoming changes here