Port gnome-terminal to GTK 4

Now that we have a bunch of performance updates landing in VTE, lets take a look at what it would take to get gnome-terminal to GTK 4 as well.

I've started a branch wip/chergert/gtk4 which gets some of the rudimentary bits in place. There are all sorts of crashes and missing things in TerminalScreen and TerminalWindow still to handle. Particularly around DnD and clipboard.

I just wanted to have an issue here for tracking/discussion as necessary.

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