MainWindow: GTK4 prep

[danirabbit]

• Bump license header
• No more decoration_layout_set in GTK4
• Explicit expands
• Child property
• More accurate cast, no more Gtk.Bin
• ScrolledWindow doesn't take args in GTK4, so use vadjustment property
• Minimize use of Gtk.Clipboard functions that are going away
• Remove set_visual. This is I think really old stuff and it's not clear it even does anything for us in GTK3

[jeremypw]

This looks good and works as expected in Horus. There are problems with OS8 in a VM but I think this is appears to be associated with the automatic creation of new workspace on fullscreening. When that is turned off the problem goes away.