frontend fixes

album artist gets populated with artist as fallback (frontend also uses
this as display name fallback for albums). added scroll position
persistence when switching main views. also added frontend cache for
faster switching.

backend/events/events.go

@@ -49,3 +49,8 @@ const (
 const (
 	LibraryConfigChanged = "LibraryConfigChanged"
 )
+
+// Library events.
+const (
+	LibraryScanComplete = "LibraryScanComplete"
+)