showTaskPane is called multiple times when switching maps

[sparkliu]

showTaskPane will be called multiple times for loading a map after a map has been switching for several times.

This is because index page register MAP_LOAD event to register TASK_PANE_LOADED event. After MAP_LOAD event is triggered several times, the same function is registered to TASK_PANE_LOADED event with exactly same number. This is not as expected. We just want showTaskPane be called once when TASK_PANE_LOADED is triggered for one time.

We should remove the MAP_LOAD event registering after TASK_PANE_LOADED is registered.

[sparkliu]

Fix patch