Texpaint refactor complete!

* Projection painting files reside in paint_image_proj.c
* 2d projection files reside in paint_image_2d.c
* Common operator/paint operation code resides in paint_image.c

All old code layout is out. Phew...Now we can at least concentrate on
each system separately when debugging this beast. We could even separate
the paint structs for 2d/projective more easily should we choose to do
so.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55171 954f8c5b-7b00-dc11-b283-0030488c597c
More UI message i18n fixes and improvements...
Fix for keyingsets tips, and make them (and a few others) findable by i18n messages extracting code (for some reasons, their bl_rna.description are void???).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55172 954f8c5b-7b00-dc11-b283-0030488c597c