[FIX] menubutton stays active after selection with keyboard

This is a fix for #22 (closed).

I noticed this issue when trying to use the xfce4-popup-windowmenu script. At first it seemed to work correctly. But i noticed that i could not trigger the menu more than one time. After some diggin and fiddling i saw that the button "got stuck" in toggled on, prohibitting it from being activated again.

This actually also was an issue without the xfce4-popup-windowmenu script. It can easily be reproduced by bringing up the menu, and selecting something with the keyboard (enter key). Now CLICK on the menubutton again, notice that you have to click it twice (one time to deactivate it).

The real issue is that the "deactivate" signal never was sent to the menu when selection with keyboard was done. replacing, gtk_menu_popdown() with gtk_menu_shell_deactivate() fixes this.