Initial hack on an underlined TabControl
Add the basic flags
Add the basic properties and create the overlay behavior for Flyouts
Remove ShowWindowButtonCommandsOnHiddenTitleBar and replace it with the new overlay behavior
Use the HasFlag method
Create the titlebar overlay behavior for all elements
Don't overlay the icon by default
Move test helpers around
Fix up the Flyout tests
Move that to Controls
Merge pull request #1459 from QuantumDeveloper/dropdownbutton

Change DropDownButton behavior. Fixes #1442
Ensure that we don't break backwards compatibility
Rename it to "WindowCommandsOverlayBehavior"
That shouldn't be here
Fix some of the casts
Merge pull request #1427 from MahApps/windowcommand-flags

TitleBar element overlay flags
Merge pull request #1468 from MahApps/fix-casts

Fix some of the casts and a request
Small refactorings
Add the TabControl underline behavior via an attached property
Of course there is room for one more TabControl!