WIP branch: http://git.xfce.org/users/eric/garcon/log/?h=gtk3

As the branch currently stands, it builds both Gtk2 and Gtk3 (both are required). Both branches get built with the same code-base there's only minor changes (GtkBox vs GtkHBox) so it should be a simple transition. To do that, both Gtk2 and Gtk3 don't use the GtkImageMenuItem anymore.

Screenshot of Garcon-Gtk2 now: app menu from panel

landry: branch builds fine as of 20160417 with gtk 3.18.

WIP branch: https://github.com/EricKoegel/garcon/commits/garcon-fixes

• Change the tests/test-display-menu to have a -gtk2 && -gtk3 version using the garcon-gtk library. This way everything can be tested.
• * TODO: Additonal features need to be added in to order to fully test out garcon.
• Merged in some of the fixes in bugzilla
• Fix menu item label alignment in gtk3.
• Ensure there are no warnings with Gtk3.20
• TODO: Additional testing before merging into master.