* 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
BKE_palette_get_active_from_context(C) now retrieves GP palettes from the active palette slot if the active object is a GP object
This makes it possible to reuse all the Palette operators as-is, without having to add a lot of GP-specific duplicates, and also simplifies much of the context polling stuff.
Code Cleanup: Move panel definitions for Palette-based colors to the Material panels file
Annotations (GP in 2D editors) will not be using this stuff, as they will not have most of the fancy drawing options (as they are based on the traditional 2D api instead).