Install shell.css

Without that, gnome-tweak-tool doesn't start

https://bugzilla.gnome.org/show_bug.cgi?id=706274
Fix vexpand/valign in shell_extensions page
Updated Spanish translation
Fix make_image definition
Updated Czech translation
Mark more strings for translate
Update POTFILES.ui
Fix search bar alignment
fix: sidebar color
Tweak name in bold
fix: section heading centered