Add Control.SetOnlyStyleClass (space-wizards#820)

This clears the collection of style classes and adds the new one. This allows controls to easily switch between several style classes without needing to keep track of the active one in their own code.

Robust.Client/UserInterface/Control.Styling.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Linq;
 using Robust.Shared.ViewVariables;
 
@@ -60,6 +60,13 @@ public void RemoveStyleClass(string className)
             Restyle();
         }
 
+        public void SetOnlyStyleClass(string className)
+        {
+            _styleClasses.Clear();
+            _styleClasses.Add(className);
+            Restyle();
+        }
+
         private void Restyle(bool doChildren = true)
         {
             _styleProperties.Clear();