Changes:
- Improve AppBarToggleButton background color when checked
- Add compact sizing support for DataGridCheckBoxColumn
- Add ability to use the assembly icon
- Improve keyboard support for Flyout and SplitButton
- Fix visual glitch when opening a CommandBarFlyout with only secondary commands
- Allow arbitrary content in TitleBarButton
- Add TitleBar.Style attached property
- Fix issue where NumberBox compact mode popup doesn't disappear and is above all windows (#13)
- Refine window style