[gtk] (9 commits) Non-fast-forward update to branch wip/baedert/for-master

[Timm Bäder <baedert src gnome org>]

The branch 'wip/baedert/for-master' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  f4031b7... window: Make decoration a widget
  4f66f3b... Window: inline function into only caller
  25e5eb8... window: Make setting the opaque region clearer
  5887f28... window: Inline update_csd_shape() into only caller
  6723f22... window: Add child widgets to a box
  364f25a... treepopover: Fix header guard
  99f4464... window: Remove public user_ref_count API
  b338c7b... widget: Remove verify_child_invariants
  6051c90... widget: Speed up verify_invariants()

Commits added to the branch:

  3d30ddd... window: Make decoration a widget
  b243b54... Window: inline function into only caller
  ca45be8... window: Make setting the opaque region clearer
  fa64416... window: Inline update_csd_shape() into only caller
  06f5a91... window: Add child widgets to a box
  b98bbf4... treepopover: Fix header guard
  3ff9388... window: Remove public user_ref_count API
  973a06f... widget: Remove verify_child_invariants
  67300d7... widget: Speed up verify_invariants()