Improve shuffle functionality

[robbevp]

I'd like to improve our current shuffle functionality, to bring it more inline with how most media players work and how the android app works.

Our current behaviour:

• Pressing shuffle is an action, with shuffles the playlist around.
• There is no way to go back to the "ordered" playlist.

In most media players:

• Pressing shuffle is a toggle, turning the shuffleMode off/on.
• When shuffled is on, there is a second playlist is with a shuffled order that gets used to determine next/prev track. (Depending on the player, this new order might be shown).
• When shuffle is off, the playlist always assumes the order in which the tracks were added.
• Depending on the player adding new tracks to a shuffled playlist adds them at the end or at a random position.

[robbevp]

I actually starting working on this, thinking it could be an easy thing to change, but due to our current implementation of the player store and component it became pretty tricky when trying to figure out all the mutations for deleting a track from the playlist, or adding (a) track(s) to it, since this would now have to mutate two arrays.