Blender Git Loki

Various fixes to get GP Palettes UI working

* Switched the UI to using the "active_gpencil_palette" (and palette color) context
vars (in favor of the "active_palette"/color) ones for retrieving the active
palette. For some unknown reason, context.active_object returns null when
trying to use it in the Properties Editor when drawing UI layouts, but it works
perfectly fine when operators calling operator.poll() or running operators.

* Sanitise many of the hacks added to the Palette operators to make them usable
for Grease Pencil-linked palettes (e.g. remove special hacky args)

* Add RNA support for getting the active GP palette slot