Introduce the new preferences dialog

This dialog replaces the existing VM "Properties" widgets entirely.

Most of this dialog uses the libhandy-1 APIs because of its decendent libadwaita, which depends on GTK 4. By the time that the Boxes display widgets are ported to GTK4, we can switch from libhandy to libadwaita with (hopefully) no trouble.

Fixes #534 (closed)

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