TitleBar element overlay flags

This PR adds fine-grained control for the overlay behavior of the `WindowCommands`, `WindowButtonCommands` and the `Icon`

- [x] Add the overlay behavior for Flyouts
- [x] Add the overlay behavior for a hidden TitleBar
- [ ] ~~Add the overlay behavor for dialogs (https://github.com/MahApps/MahApps.Metro/issues/1272)~~ We can do this later
- [x] Ensure backwards compatibility
- [x] Add a crapload of tests

This is achived with an enum with the following values:

- `Never`
- `OverlayFlyout`
- `OverlayHiddenTitleBar`
- `Always`