Update gtk-sharp v3 libraries to those distributed by ubuntu
Add RadioToolItem to allow for separate radio buttons on the toolbar, similar to OS X's properties dialogs, etc.  Only one radio group per toolbar.
Properly implement CheckToolItem on all platforms, Fixes #199.
Mac: Fix SeparatorToolItem when using divider style (now just shows a small space, since it is deprecated on OS X).
Update test app to use check, radio, and separator tool items.