improved library scan speeds, added library manager component

-libraries tab now opens a library manager
-choose library directory, manually soft scan and rescan
-batched db queues
-mp3 header-based duration extraction

backend/events/events.go

@@ -56,5 +56,6 @@ const (
 
 // Library events.
 const (
+	LibraryScanStarted  = "LibraryScanStarted"
 	LibraryScanComplete = "LibraryScanComplete"
 )