Adding the Loop functionality, along with the Player changes to support
it, and a button to the player controls.
`Player` now has a `loopMode` attribute, which determines whether it
should loop. At the moment, only the following modes are supported:
* `LoopNone`: Disables loop.
* `LoopAll`: Enables loop for the entire playlist queue.
This has been designed as an enum, to support other modes in the future
(e.g. loop for the current track only).
It depends on the `loop` functionality provided by MPV (using the
[`loop-playlist` option](https://mpv.io/manual/master/#options-loop-playlist)),
so no custom logic is needed to reset the currently playing index.
This reverts commit cd52a893d4, which introduced a regression.
Pages with a tracklist may not have the same in-memory model of the track,
so it is necessary to directly inform them of play count updates.
Michael Manganiello