New Option(s) to Specify Position of Popout Player Window

[rthaut]

From https://addons.mozilla.org/en-US/firefox/addon/youtube-popout-player/reviews/1713749/:

[Add a feature] to be able to place the popup when choosing a custom size.

[rthaut]

In theory this functionality should be available (and optional) when using the auto size or when specifying a size.

It also should accommodate/respect #285. Most likely users would want to either specify the position manually, use the last position, or just let it auto position.