UI translation from inside Blender UI: second part.

This is the py addon to use for embeded Blender ui translation. It can edit various UI elements' messages and tips, save the changes to the relevant po, "compile" a new mo placed into user's datafiles dir, and erase that same files (to get back "official" translations).

Notes:
* Still work in progress. Most likely some remaning bugs (even though it works quite well for me)...
* All UI elements in Blender aren't translatable this way (e.g. panel labels just don't react to right mouse clicks currently...)
* RTL languages won't work as well as others, still have to implement some kind of revert-RTL process...
* Having a nicer way than editing user_settings.py in modules/bl_i18n_utils to set up needed parameters is mandatory! Will try to use some kind of user preferences in user's config dir.

And a whole doc to write! :/