[expander] Notify "expanded" property when changed using mouse.
[expander] Notify "expanded" property when changed using mouse.

Previously the notification was only emitted when the property
was changed by code.

clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx