Various small enhancements/fixes.

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...

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4333 96b784f8-4ae3-dd11-aff7-0030488c597c