Add a bunch of unit tests
[GTK] Fix spinner

The GType property was missing
[GTK] Fix NRE in SegmentedButton when there are no buttons
[GTK/Paned] Fixed margin test failure
ScrollView children can have margins

So the backend needs to implement IChildPlacementHandler
Enable ComboBoxEntry tests
Cosmetic fix in test
[Gtk/SegmentedButton] Add missing allocate call

base.OnAllocate always needs to be called
[Mac] Fix widget placement updates

Properly change the widget placement when the margin or
any of the other placement properties change.