[expander] clean up setting the expanded property
[expander] clean up setting the expanded property

Allow the set_expanded function to set the value of expanded property
explicitly.

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