LibreOffice GTK4 Port: Some more progress

More progress, Entries, SpinButtons, PushButtons, CheckBoxes/RadioButtons, TreeViews and basic Notebooks and DrawingAreas working in the GTK4 port

First Cut at a GTK4 Port of LibreOffice

Early days yet, but here's some screenshots of an experimental port of LibreOffice to GTK4 I've commited upstream during the week. It starts without crashing, renders, accepts user keystrokes and mouse events, basic Input Method support also works.

Standard GTK file dialog functionality works.

Standard GTK message dialogs work.

Context menus, now using Popover Menus, work.

Toplevel Menubar and menus, via gtk_popover_menu_bar_new_from_model, are also functional.

Working sample native GTK dialog. A work in progress, my current thinking is with GtkButtonBox gone that moving the old action area contents to the dialogs GtkHeaderBar seems the easiest route.