Merge branch 'master' into GPencil_Editing_Stage3

Revert previous eraser tweak - It made using the mouse to erase too difficult

GP Sculpt: Cleanup old comments and some debug prints

GPencil EditMode Keymap: Asterisk/Star Key on Numpad can be used to toggle Isolate Layers

GPencil Eraser: Some tweaks to eraser strength

Damping factors make the eraser too hard to use in general cases, so leaving as-is.

Fix: Header help text was not defined for Grease Pencil "Poly" drawmode

GPencil UI: Added a few extra icons for decoration

GPencil EditMode Keymap: Alt-C now activates the "Convert" tool for Grease Pencil (instead of the object-mode one)

Code Cleanup: Remove code for old GP eraser

GPencil Eraser: Eraser is now pressure sensitive

Instead of immediately erasing points, the eraser now first reduces the thickness
of the stroke points before culling the ones that have become overly thin (and
cannot be rendered properly anyway as a result). It is also pressure sensitive now,
and has a linear falloff effect (i.e. points further away from the center of the
eraser circle get affected less). This should make it easier to use to eraser to
make fine adjustments to your sketches.

Notes:
* The eraser has been rewritten to use a new algorithm. Specifically, it now takes
advantage of the stroke point deletion methods developed for the Delete operator
instead of it's own buggy split-and-delete methods (which were only removing
a single point from each stroke each time).

* In tests so far, it seems to allow a wider range of control over what gets erased
and what doesn't. Pressing hard (or using a mouse) should still just erase everything
with relative ease still.

* The way that this behaves could still do with some tweaking. In particular, care
is needed when trying to use the eraser to "lighten up" tips of thin bunches of
strokes (e.g. eyebrows/moustaches), as it still tends to gobble these up too much.
We could probably do with some tool settings to control the eraser strength.

Fix: Delete tagged GPencil stroke points now adjusts timing info

gp_stroke_delete_tagged_points() now adjusts timing data for stroke points
to ensure that all the timing info will be valid after creating the new
stroke segments (from splitting the original stroke).

Several other tools need to be modified to do this still (e.g. Copy and Duplicate)

GPencil: Code cleanup - Moved logic for removing selected/tagged points from strokes into a separate function

Compile fix for changes from master

Merge branch 'master' into GPencil_Editing_Stage3

Conflicts:
source/blender/editors/transform/transform_manipulator.c

Code Cleanup: Move away from inlined math for the eraser tests

Some code cleanup - Using proper types in more places

Fix: Fixed incorrect tooltip

This tooltip was missed when was copying over the RNA stuff from my
earlier Pose Sculpting work (that much of these tools were based on)

GPencil Sculpt: Expose setting for toggling whether to invert the way the brush works

As in mesh sculptmode, there is now an Add/Subtract toggle to control whether the
brush applies the named effect or not (e.g. Thickness = Thicker/Thinner, or
Pinch = Pinch/Inflate)

Fix: Keep onion skinning toggles in sync

When toggling the per-layer onionskinning settings, the datablock-level
toggle (shown on the header) should also be updated. This only really
applies when there is/was a single layer with onionskinning.

GPencil Eraser: Operates on all visible + editable layers instead of active only

The GPencil eraser now operates on all visible and editable layers, instead of
only operating on the active layer. There was really no reason why it needed to
only consider the active layer, as doing so only made it confusing for the user when
the strokes they were trying to erase wouldn't be affected. Besides, if you really
needed to restrict the eraser to working on a particular layer, you could just
lock all the others (which is now very easy with the new operators)