Create Playlist Save Preferences

This patch solves https://github.com/clementine-player/Clementine/issues/4480. It adds three options for playlist paths: Automatic (current behavior), Absolute, and Relative. It also has a Write Metadata option that can be turned off.

The options have been implemented in XSPF and M3U. I'm not knowledgeable about the other types, so I didn't try, but the infrastructure is there.