Toggle LibraryProvider availability on authentication status

- Add Visible property to LibraryProviders
 - Filter LibraryProviders based on .Visible
 - Add authentication status to ChangeCloudSyncStatus calls

Library/Provider/LibraryProviderFileSystem.cs

@@ -78,6 +78,11 @@ namespace MatterHackers.MatterControl.PrintLibrary.Provider
 			GetFilesAndCollectionsInCurrentDirectory();
 		}
 
+		public override bool Visible
+		{
+			get { return true; }
+		}
+
 		public override void Dispose()
 		{
 			directoryWatcher.EnableRaisingEvents = false;