[gtkmm] (2 commits) Non-fast-forward update to branch wip/dboles/builder-retval-4

[Daniel Boles <dboles src gnome org>]

The branch 'wip/dboles/builder-retval-4' 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:

  d135aa0... Builder: Fix wrong use of @retval for output args:

Commits added to the branch:

  ff3e8dc... tests/builder: Drop .ui properties gone in GTK 4 (*)
  bf1ce80... Builder: Fix wrong use of @retval for output args:

(*) This commit already existed in another branch; no separate mail sent