i18n: Add special case for gl language name and update translators.xml
Remove commented-out custom drawing for header toolbar

The toolbar looks good with the default Adwaita theme in GNOME 3.8, so
we won't need any custom drawing anymore.

With earlier versions of GNOME 3, there was an ugly border around the
toolbar, so I had kept the custom drawing code from GTK 2, in case we
needed it again.