Improved Widget.RemoveChildren performance

Modifying the list potentially several thousand times is really slow, so notify the child elements that they are being removed and then clear the list in one go.

OpenRA.Game/Widgets/Widget.cs

@@ -524,8 +524,10 @@ namespace OpenRA.Widgets
 
 		public virtual void RemoveChildren()
 		{
-			while (Children.Count > 0)
-				RemoveChild(Children[Children.Count - 1]);
+			foreach (var child in Children)
+				child?.Removed();
+
+			Children.Clear();
 		}
 
 		public virtual void Hidden()