LibGUI+WindowServer: Add support for GWidget tooltips.

Any GWidget can have a tooltip and it will automatically pop up below the
center of the widget when hovered. GActions added to GToolBars will use
the action text() as their tooltip automagically. :^)

Servers/WindowServer/WSWindow.cpp

@@ -107,6 +107,8 @@ static WSAPI_WindowType to_api(WSWindowType ws_type)
         return WSAPI_WindowType::WindowSwitcher;
     case WSWindowType::Taskbar:
         return WSAPI_WindowType::Taskbar;
+    case WSWindowType::Tooltip:
+        return WSAPI_WindowType::Tooltip;
     default:
         ASSERT_NOT_REACHED();
     }