MediterraneanNight: Implement murrine-{dark,menubar} to please GTK.

Some of this was shamelessly stolen from some forums, namely:

* https://bbs.archlinux.org/viewtopic.php?pid=1026506#p1026506
* https://forum.xfce.org/viewtopic.php?pid=28950#p28950

I am sure that not everything here is strictly needed (in the sense that I
must be including useless code that should not be here), but I would like to
receive some peer reviews before removing parts of what is here.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>