Add a hook for widgets being removed, and ensure widgets are removed consistently.

OpenRA.Mods.Cnc/CncLoadScreen.cs

@@ -105,7 +105,7 @@ namespace OpenRA.Mods.Cnc
 
 		void TestAndContinue()
 		{
-			Widget.RootWidget.RemoveChildren();
+			Widget.ResetAll();
 			if (!FileSystem.Exists(Info["TestFile"]))
 			{
 				var args = new WidgetArgs()

OpenRA.Mods.Cnc/Widgets/Logic/CncIngameMenuLogic.cs

@@ -42,7 +42,7 @@ namespace OpenRA.Mods.Cnc.Widgets.Logic
 				world.WorldActor.QueueActivity(new CallFunc(() =>
 				{
 						Game.Disconnect();
-						Widget.RootWidget.RemoveChildren();
+						Widget.ResetAll();
 						Game.LoadShellMap();
 				}));
 			};