Assamese translation reviewed
po/vi: import from Damned Lies
Updated Vietnamese translation
Hardcode colors for all backgrounds

If we don't add scolor and pcolor, the background panel shows
color swatches which are useless and distracting for these
backgrounds.

https://bugzilla.gnome.org/show_bug.cgi?id=675466
Refreshed set of extra wallpapers for 3.6
fix typo, lower quality to decrease extensive filesize
darken UnderLit
update metadata
Assamese translation updated
Updated Serbian translation
More cleanup for 3.6

- lower redundancy, provide more variation
correct author's name
Updated Galician translations
one last wallpaper to fill the grid.
3 more wallpapers by Garrett.
fix scaling mode.
Updated Spanish translation
Updated Traditional Chinese translation(Hong Kong and Taiwan)
Add INSTALL file to MAINTAINERCLEANFILES
Switch to xz compression by default