Add a placeholder in the computer list if none are found. Retry service discovery if it fails.

Limelight/Utility/Computer.m

@@ -28,4 +28,13 @@
     return self;
 }
 
+- (id) initPlaceholder {
+    self = [super init];
+    
+    self.hostName = NULL;
+    self.displayName = @"No computers found";
+    
+    return self;
+}
+
 @end