artwork: refresh artwork checks default image, free allocations at shutdown
coverart: remove fprintfs from testing
artwork: add artwork.h to the install list
artwork: remove restriction on file mask length, speed up DB_EV_CONFIGCHANGED when nothing has changed
artwork: don't invalidate the whole cache just for default artwork change
Merge pull request #59 from Lithopsian/search

artwork.h into install list, improve pref handling and updates