Add basic video queue system

This MR adds a basic video queue system, where you can play videos like normal (which replaces the whole queue with the one video) or add more videos to play next. Autoplaying will come in the next MR, but you can also hit "next" in the mini player to do the same. A visual representation of the current queue is located above the recommended videos.

The UI is not final of course, it will be improved before release.