[gtkmm] (5 commits) Non-fast-forward update to branch wip/dboles/builder-nicer-4

[Daniel Boles <dboles src gnome org>]

The branch 'wip/dboles/builder-nicer-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:

  4ab8c58... Builder: Fix wrong use of @retval for output args:
  1504d8b... Builder: Add returning vers of get_widget|_derived
  68288a6... tests|gtk-demo: Use new Builder.get_widget*() vers
  9962c28... Builder: Add get_object<Derived>(name) overloads

Commits added to the branch:

  a8bb760... Builder: Document get_widget_derived() builder arg (*)
  495cc8d... Builder: Fix wrong use of @retval for output args:
  940f06a... Builder: Add returning vers of get_widget|_derived
  cb88060... tests|gtk-demo: Use new Builder.get_widget*() vers
  be24e10... Builder: Add get_object<Derived>(name) overloads

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