Merge pull request #159 from GuillaumeGomez/master

Add widgets
changed ModifierType to bitflags, more rust-like, and added a usage example to examples/gtktest
Support the Quartz backend on OS X
Remove the Socket widget on non-Linux OSes
Remove unnecessary #else condition
fix Modifiermask value

I believe it should be this value..
https://fossies.org/dox/gtkmm-3.14.0/types_8h_source.html
convert modifierintent to bitflags
make modifiertype used by GDk structs
Merge pull request #160 from buster/modifier-enums-reform

changed ModifierType to bitflags, more rust-like, and added a usage exam...
Remove X11 pkgconfig path, and allow cross compile so find_library works
Merge pull request #161 from oakes/quartz

Support the Quartz backend on OS X
Beggining of gdk::Window binding