gtk-murrine-engine: New formula

Added gtk-murrine-engine formula: a GTK2 engine that features a glassy
looks. This formula is written based on gtk2-murrine MacPorts Portfile (https://trac.macports.org/browser/trunk/dports/gnome/gtk2-murrine/Portfile).

Closes #24289.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>