Log Message:
gtkmm3: updated to 3.24.4

3.24.4 (stable):

The tarball for 3.24.4 has been created with 'meson dist'.
If you build with Autotools from the tarball, please read the relevant
part of the README file.

Build on Windows:
* Gtk::TreePath: Export comparison operators

3.24.3 (stable):

The tarball for 3.24.3 has been created with 'meson dist'.
If you build with Autotools from the tarball, please read the relevant
part of the README file.

Gtk:
* Actionable::get_action_target_value(): Fix ref count

Documentation:
* Improve the documentation of Gtk::Application::add_window()
  and Gtk::Window::set_application()
* Gtk::Button::set_image(): Fix the documentation
* Gtk::TreeView::append_column(): Add code example
* README: Add instructions for building gtkmm
* Explain key values

Demos:
* Don't use C++14 auto deduction
* Update the GLArea demo (similar to updates in gtk+)
* GLArea demo: Fix a crash

Tests:
* Builder test: Test derived props declared in C++

Build:
* Improve NMake support
* Fix Autotools build with X11 backend disabled (--enable-x11-backend=no)
* Use __declspec(dllexport) when building with Visual Studio
* Add support for building gtkmm with Meson
* docs/reference/: Update for Doxygen >= 1.8.16