Draft: window: Embed transient windows into small parents
This prototype implements embedding transient windows into their parent windows when it is small (e.g. on mobile).
finish converting from the
- close the whole window with the close button (if feasable)
- back (close) button (see #195 and :
- allow to mark headerbars as "main" so they can receive a back button or
- give enough info to apps so they can know when to show their own back button
- anyyhing I forgot?