baseclass: Add is_visible

Remove the duplicated implementations everywhere

virtManager/manager.py

@@ -221,13 +221,11 @@ class vmmManager(vmmGObjectUI):
         if self._window_size:
             self.config.set_manager_window_size(*self._window_size)
 
-    def is_visible(self):
-        return bool(self.topwin.get_visible())
-
     def set_startup_error(self, msg):
         self.widget("vm-notebook").set_current_page(1)
         self.widget("startup-error-label").set_text(msg)
 
+
     ################
     # Init methods #
     ################