Updates

Here's a breakdown:

- Window.WindowStyle/SendToBack/BringToFront/TopMost fully implemented + test
- Window.GotFocus now used instead of Activated event (and LostFocus fires when losing focus/deactivated)
- KeyUp/Down events are now fully implemented across platforms
- Key.IsControl/Alt/ShiftKeyUp() methods replaced by more generic IsKeyUp/Down(key, modifier)
- Remove Icon.FromEmbeddedResoure to load icons from .resx files
- More Mac updates for TextureBrush/LinearGradientBrush to make them use the current transform instead of ignoring it, making it behave like other platforms
- Added RadioButtonList/EnumRadioButtonList
- More stuff/bug fixes