Artwork: reset, shutdown, fixes, and scanning zip files

Disconnect coverart cleanly before trying to stop plugins, but all still in gtkui_plugin->stop().
Made artwork_reset faster: makes all callbacks, but may leave a fetch still running.  Terminating the thread may still be slow.
Cache cleaner thread terminates more quickly when doing a scan.
Can scan for artwork inside VFS containers (vfs_zip->scandir() fixed to filter like native scandir, other archive plugins won't work yet).
Removed the refresh cache preference button, doesn't work smoothly.