Remove file check for pytmdb3.cache

This is a partial roll back of 2c50a824b and remove the check for the
pytmdb3.cache file, allowing the library to simply create the file if
necessary.

Fixes #11876
MythMusic: allow some views to be nested so you can return to previous views

This allows you to for example switch to the full screen visualiser from the
playlist and playlist editor views and when you exit the visualiser you will
return to the previous view you was on rather than exit MythMusic.
MythMusic: always start playback after adding tracks to the current playlist