Log Message:
gtkmm4: updated to 4.16.0

4.16.0 (stable) 2024-09-10

Almost identical to 4.15.1.

Gtk:
* gtkmm.h: Add eventcontrollerlegacy.h.

4.15.1 (unstable) 2024-08-30:

Gdk:
* DrawContext: Deprecate begin_frame(), end_frame(), is_in_frame(),
  get_frame_region().
* Surface: Deprecate set_opaque_region().
* Cursor: Rename create(const SlotGetTexture& slot, .....).
  Call it create_from_slot(). It was added in 4.15.0 (unstable),
  but some create() calls in old code became ambiguous.

Gtk:
* Window: Add set_interactive_debugging().
* TextBuffer: Add enum TextBuffer::NotifyFlags,
  add_commit_notify(), remove_commit_notify().

Build:
* Require gtk4 >= 4.15.5.