Figure out what picture-in-picture features are broken or missing

We currently exclude browser/extensions/pictureinpicture/ from our build, whilst including toolkit/components/pictureinpicture/.

The feature seems to work, with maybe the urlbar button missing (I'm not 100% sure about when it is meant to appear).

We should figure out what browser/extensions/pictureinpicture/ does and whether we want to include it in our build.