Blender Git Loki

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