GTK-Quartz and Mac integration support

 * Added option to link against GTK-Quartz instead of GTK-X11 on OSX
 * Added `gtkmacintegration.nim`, a wrapper for the [GTK Mac integration][1]
 * Added an example for using GTK Mac integration
 * Added `g_object_new` to `glib2` because it was missing for some reason and GtkOSXApplication needs it

Builds on other platforms aren't affected by these changes. GTK Mac integration requires GTK-Quartz. Linking against GTK-Quartz has to be explicitly enabled with `-d:gtk_quartz`.

 [1]: https://github.com/jralls/gtk-mac-integration