meson-templates: added Gtk4 C template
Introduce the first Gtk4 C template. Reworked also the example shown. It contains a application menu, shortcuts window, about dialog.
I left everything in there in order to introduce Gtk4 templates for additional languages. Therefore it is only necessary to add the language to support the template-category and then you can upcast the Gtk+-3.0 template to Gtk4. I hope this is okay @chergert.