icons, pixmaps and UI files should be moved to a data/ directory in order to separate resources files from source code.

I have a working branch over there: https://github.com/stephh/PiTiVi/tree/datadir, please check it out, merge and report any things I might have forget.

Here's a little summary of the changes, I would be glad to hear your feedback about it:
- /pitivi/pixmaps -> /data/pixmaps
- /pitivi/ui/*.xml|glade -> /data/ui/*.ui
- /icons -> /data/icons
- pixmaps and UI files are now installed in /usr/share/pitivi, all together, like most programs (I think)
- configure.py.in gets a get_data_dir() and get_ui_dir() method ;
- get_global_pixmap_dir() is gone, not used in the code (except some win32 duplicate file, grep for it in master)
- All UI file now use get_ui_dir() properly, so does glade.py. I deleted some win32 .exe check, because a) easier to deal with, b) this should be done in configure.py.in anyway, right?

This is all I can think of right now, make sure to look at the branch and the git log/diff. Thanks!