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.
Added more tests to verify widget placement
Minor fix when calculating widget placements
[Box] Properly align widgets inside the allocated slot
[Mac/Frame] Fix child allocation issues
[Mac] Implement DatePicker backend
Split the Slider control in VSlider and HSlider
[Mac] Implemented Slider
[Mac] Fixed scrollbar orientation
Track api changes in Slider API
Added Slider and DatePicker samples
[WPF] Fix NRE in ScrollView