Log Message:
Update gtkmm3 to 3.22.0

Highlights:

* FileChooser: Added add_choice(), remove_choice(), set_choice(), get_choice().
* Menu: Add popup_at_rect(), popup_at_widget(), popup_at_pointer(),
  signal_popped_up(), and the anchor_hints, rect_anchor_dx, rect_anchor_dy,
  and menu_type_hint properties.
* Popover: Add popup() and popupdown(),
* RadioMenuItem, RadioToolButton: Add join_group().
* ScrolledWindow:
  - Added get/set_max_content_height/width() and properties.
  - Added get/set_propagate_natural_height/width() and the properties.
* Add the ShortcutLabel widget.
* TreeNodeChildren: Deprecate the reverse iterator.
* Pixbuf: Added set_option(), remove_option(), and copy_options().
* PixbufFormat: Added is_save_option_supported().
* GLContext: Add set/get_use_es()
* Add Monitor.

Full list of changes at:

  https://git.gnome.org/browse/gtkmm/tree/NEWS?h=3.22.0