Blender Git Loki

Annotations: Dynamically updating layer-color preview icons + "Active Note" enum

This adds a new enum with dynamically-generated icons to show previews
for the icons associated with each GP layer used for annotations. This
enum is used as part of the UI for annotations (specifically, in the
topbar settings for the active tool), making it easier for users to
see what color strokes drawn using the active note/layer will look like.

I ended up needing to create a new icon type in BKE_icons.h so that
we can store references to the layer/color used.

Known issues:
* The icon seems a bit too big, and would look a bit nicer with
rounded corners. But there are too many issues with the UI roundbox
functions (e.g. _aa() makes colors too dark, and non-aa versions look
too rough).

* Changing layer colors doesn't redraw the icons. A manual refresh
(by mousing over the widget) is needed.