Make Gnome3 be bearable

The layout of Subsurface on Gnome 3 had a few flaws, since Qt and Gtk
are not really compatible. This implements a CSS that makes the use
of Gnome 3 more pleasant to the eyes.

Fixes #318

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>