Hack to make the subsurface work on Gnome3 shell

The Gtk+ style on the Gnome shell is somewhat broken on Qt for
some reason. This hack pokes the system, checks if it's running
gnome-shell, and if the current style is gtk+ ( I couldn't just
check for gtk+ since it worked on XFCE and other Gtk based enviro
ments. so a double check is needed. ) then I changed the Pallete
of the affected widgets by hand.
not a pretty hack but worked.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>