GdkWin32Monitor fixes

Merged Jason Francis requested to merge jf/gtk:win32-monitors into main

I was using this program to test out on a laptop: jf/gtk$5783

And I got some weird behavior, GDK still saw the internal laptop screen after I turned it off, and it wasn't fullscreening on the monitor anyway.

This MR correctly removes disconnected monitors from the ListModel, also makes gtk_window_fullscreen_on_monitor work on Windows.

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