Fix WindowCommand brushes when using Flyouts

Now with 95% less bugs!

This PR changes the Flyouts so that they are "dumb" and let the host window handle all the color stuff

This fixes the following things:

- A dark flyout doesn't have dark window commands anymore (lol how did I miss that?)
- Only the topmost Flyout changes the WindowCommands, if it is closed, the WindowCommands adapt to the underlying Flyout
- Theme changes are respected
- Closes #947

What this doesn't fix:
- My sleep cycle apparently, it's 4 A.M here...