Remember changes to song source between sessions.

[BloodRaven0]

When you change the source of a song, it should always play from that source in the future, regardless if the program is closed / reopened. It currently forgets this, so if a song source defaults to a live or a music video with background noises, if I switch it to the "official audio", then force close Spotube and reopen it, it will again play the default source, which is the live.

I propose that user-switched sources be saved as a sort-of cookie file locally and referenced when a song is played to find the user-preferred source. I am no programmer, so there might be a more elegant solution.