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???).
Most notable difference from now on will be that all py is handled from current blender's resource dirs, no more from source dir. Better for consistency, and avoid e.g. cycles' addon to be checked twice...
Notes:
* Everything is still a bit raw and sometimes hackish.
* Not every feature implemented yet.
* A bunch of cleanup is still needed.
* Doc needs to be updated too!
