In Windows builds of GTK+ 3.x, the dll filename is libgtk-3-0.dll.
We use this opportunity to use a common const in the DllImport
statement for all custom code.
The Group property for all the Radio* classes (RadioAction, RadioButton,
RadioMenuItem and RadioToolButton) was exposed as a GLib.SList,
with its elements untyped.
We now hide the various Group properties and have custom code to handle
them correctly.