Ensure CollectionList cache is saved on shutdown.

JuK caches its list of collection list items for re-use on subsequent
startup, to reduce the time needed to wait for Taglib to rescan music
files that have been previously scanned (and presumably have not changed
outside of JuK). It also records the list of playlists on shutdown, including
playlists which are not accessible as M3U files.

Shutdown behavior has long been problematic for JuK as a Qt5 app due to
a) poor coding and b) changes in how close/quit event handling has
evolved since it's roots in KDE 3. I made some attempt at cleaning this
up with commit a951caf3e9dd54f56fa1a32d4dc8bbbdfc269252, but that bug
fix fell short, as it did not ensure that JuK caches the collection and
records the list of playlists.

Most users would experience this as "merely" a performance hit on
startup.

The fix is to ensure the caching/state-saving code is called (as part of
GUI class destructors) by destructing the PlaylistSplitter class, which
had almost been working prior to a951caf.

I did not see a directly applicable bug report.

CHANGELOG:Speedup JuK restarts by fixing bug preventing collection data from being saved on shutdown.
FIXED-IN:19.08.2