Configuration UI bug fix

Toggle Enabled/Disabled for the countdown spinBox
API setVolume

API updated adding setVolume(volume) and allowing arguments for volume
up/down
Removed test methods authAsGuest/DJ
API: added getCollectionAllTracksJSON()

Export all tracks, defaults to 500 tracks max
API debug: clean escape back-slashes using regexp
API added getCollectionInfoJSON()

Get Collection stats: totalTracks/albums/artists/genres and last update
date
API: appendPlaybackInfo()

Add volume and engineState information to playback commands response
API output cleanup: use args{} and playback{} in result
API export full library

added getCollectionAllAlbumsJSON() and getCollectionAllGenresJSON()
Cleanup on getCollectionTracks(): display actual total, remove
artist/album/genre string values (names)
API: include empty entries

Return null values for artists, genres and albums instead of skipping
Version 1.6 relase