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?