Add a way of selecting the monitor used to display fullscreen WebViews

[rdw-software]

By default, the code for #51 seems to select the primary monitor (at least on Windows). While this is fine, it would be useful to make this configurable since there's no guarantee it'll always be the one you want. In fact, GTK doesn't do it (inside WSL at least).

Prototype (GTK): https://gist.github.com/Duckwhale/1a243f579db713c73d790d29eef4c76b

Doesn't seem to work inside WSL either, so I'll need to test on my laptop (with a second monitor installed). Deferred until later.

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Follow-up to #51