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).

To do:

• finish converting from the set-transient-window prototype to transient-for
• 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?

Fixes #52

Relevant issues:

• #195
• #315 (closed)