Windows: Make UI update significantly cheaper.

Setting menu strings, reloading shaders, and redrawing the menu were all
a bit slow.  This skips those things unless they've changed.

Windows/MainWindow.cpp

@@ -863,8 +863,9 @@ namespace MainWindow
 			break;
 
 		case WM_USER_UPDATE_UI:
-			// This also calls ChangeMenu
 			TranslateMenus(hwndMain, menu);
+			// Update checked status immediately for accelerators.
+			UpdateMenus();
 			break;
 
 		case WM_USER_WINDOW_TITLE_CHANGED:
@@ -881,9 +882,7 @@ namespace MainWindow
 
 		case WM_MENUSELECT:
 			// Called when a menu is opened. Also when an item is selected, but meh.
-			// This is a good place to update the enabled/disabled state of menu items.
-			UpdateMenus();
-			SetIngameMenuItemStates(menu, GetUIState());
+			UpdateMenus(true);
 			WindowsRawInput::NotifyMenu();
 			break;