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 a951caf3, 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 a951caf3. 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