Support compiling against GTK-Quartz
Added GtkOSXApplication support

 * added gtkosxapplication.nim, which wraps GtkOSXApplication
 * added an example that shows how to use it
 * added g_object_new to glib2 because it was missing
Renamed `macosx_quartz` to `gtk_quartz`

 * `macosx_quartz` is too broad and a poor choice for a global symbol.
Merge pull request #1 from flyx/master

GTK-Quartz and Mac integration support