Blender Git Loki

Initial/backend support for animating settings of Grease Pencil datablocks

This commit makes it possible to insert keyframes for the line thickness,
opacity, colour, and even volumetric/non-volumetric options. It is useful
to be able to do this in order to control the thickness of lines when the
camera is placed quite close.

Notes:
* Currently, apart from the keyframe status indicator colours applied to the
buttons, these keyframes are not shown anywhere. Support for this will be
added to the animation editors shortly, but for those who need this functionality,
it's better that it works in some way first :)
* No support for drivers exists yet. This may still get supported a bit later...