robot state display: subscribe on enable / unsubscribe on disable

The display didn't connect in onEnable.
It only did after the user changed the topic name.

Also, disconnect onDisable. There's no need to keep listening to the topic
until the display is enabled again.

moveit_ros/visualization/robot_state_rviz_plugin/src/robot_state_display.cpp

@@ -377,13 +377,15 @@ void RobotStateDisplay::onEnable()
     robot_->setVisible(true);
   }
   calculateOffsetPosition();
+  changedRobotStateTopic();
 }
 
 // ******************************************************************************************
 // Disable
 // ******************************************************************************************
 void RobotStateDisplay::onDisable()
 {
+  robot_state_subscriber_.shutdown();
   if (robot_)
     robot_->setVisible(false);
   Display::onDisable();