support for multiple instances for multi monitor configurations

[Piranna]

When having a multi monitor configuration and you try to use several instances of indicator-appmenu-applet, it crash and gets useless and start eating memory. Correct behaviour should be:

1º don't crash when multiple instances are running, both on the same or diferent panels, both on the same or diferent monitors
2º get all instances updated with the content of the first plane application
3º ideally, get each instance updated with the content of the fist plane application on that monitor. Task switcher applet has some functionality regarding to detect this, being able to show only the windows on the current monitor, also when moving them from one to another.