Blender Git Loki

This commit replaces old "manual" Editors menu with nice RNA-based one (so once again, some cleanup of old UI code). It makes the following changes:

* RNA's editor types enum (space_type_items) has been re-ordered, added icons, and removed internal-only SPACE_EMPTY value, so that it matches the menu.

* Two fixes in code drawing enums as dropdown menu:
** All items were taken into account when computing the number of needed columns, now simple separators (void string items) are ignored.
** Simple separators items were drawn as labels, taking far too much space!

And now translators will be free from that ugly, insane menu-string! :P

Thanks to Brecht for the review.