Blender Git Loki

Fixes for various problems with the linked-datablock handling code for GP features

* The new "Scene->gp_object" field was not being relinked in various places

* The "bGPDstroke->palette" relinking calls were missing the standard "id.lib" args

* "bGPDlayer->parent" should be handled in lib_link_gpencil() not direct_link_gpencil(),
It was also not handled in the expand_doit()

Code Cleanup - Some leftover cruft/reshuffling/formatting

Palette Slots - File IO and Basic RNA support

Palettes Slots - Fix incomplete DNA changes from previous commit

Merge branch 'blender2.8' into greasepencil-object

GP Palette Slots - Initial DNA structures

Start of support for GP "Palette Slots". These act like "Material Slots"
in that they allow a single GP datablock to refer to multiple Palette
datablocks, making it easier for the user to manage the set of available
palettes.

Code Cleanup - Clarifying names, comments, and shuffling fields for clarity

Code Cleanup - Move all old/deprecated bGPDpalette stuff together to be easier to remove/disable in future

Code Cleanup

Build fixes after merge

iroundf -> round_fl_to_int

Merge branch 'blender2.8' into greasepencil-object

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

Code Cleanup: Move 2D Monkey code to its own file in editors/gpencil

Fix: The code to reset GP Objects to Object Mode on load wasn't working

UI: Remove Palette Panel header

Merge branch 'blender2.8' into greasepencil-object

Code Cleanup: Replaced inlined layer-copying code with a call to the copy_ex() method

Fix: Make Single User wasn't supported for GP Objects

Fix: Doing a Scene "full-copy" wouldn't relink the gp_object

Fix: When transforming GP strokes in "Local" mode, the strokes would get obscured by
the transform constraint lines

Fix: If the active object is not a GP object, starting to draw
with D-LMB would create a new GP object

This was behaviour was bad if you were trying to use GPencil for
anything other than 2D-3D art (e.g. for creating freehand input
for addons/Pose Sketching, annotating your work in EditMode, etc.)
as you could easily end up with all your annotations/sketches in heaps
of separate GP objects.

This commit improves the sitation slightly, by introducing an
"GPencil Object" property to Scene (similar to the existing "Active Camera"
and "Active Clip" settings).
* The idea is that the first GP object added to the scene as the "default
GPencil Object".
* Everytime you start sketching with GPencil AND the active object isn't
a GPencil Object, this "default GPencil Object" will become the active object,
and any strokes you draw will be added to this object (instead of a new
GP object being created)

TODO:
* Investigate if we can do this without losing whatever mode the user was
in previously. This new workflow makes GPencil a lot less useful for a
bunch of different annotation/addon workflows it could be used for in 2.7