Implement double click to enqueue view as an option. (Based off of #362)

Based off of @ngraham's merge request !362 (closed)

I wasn't sure what the best way to contribute in this case, so I failed at git for a while until I managed to add my changes on top of Nates. So apologies for any git nonsense.

Basically the additional code simply passes the double click signal to the parent, and calls their enqueue function. There's some code to switch the track after it enqueues, it's sort of shoved into genericEnqueueToPlayList since the view/playlist enqueuing happens async, so I'm open to suggestions there. I've also fixed the option not loading on startup/config change.

Currently there's a small issue with the VLC backend when you enqueue view/change tracks that outputs:

[00007ffabc000e00] cache_read stream error: cannot pre fill buffer
[00007ffabc0076f0] mjpeg demux error: cannot peek

I assume this is because Elisa/VLC backend is loading the first track on the playlist, and as that starts we interupt it with a new track. This does not affect media playback, and this also does not affect the QT multimedia backend. Any ideas/suggestions on how to resolve that would be wonderful!

Cheers,

Mel