core: Tidy up multi-valued property update functions
core: Tidy up multi-valued property update functions

Create a new _update_multi_valued_property() helper function and
corresponding delegates, to mirror the existing
_update_single_valued_property() helper.

This new function is now used for most multi-valued property updaters in
Individual. It’s not used for groups, because they have weird signalling.

pwithnall/folks,renatofilho/folks,renatofilho/folks,pwithnall/folks,GNOME/folks,GNOME/folks,renatofilho/folks,renatofilho/folks2,pwithnall/folks,GNOME/folks,renatofilho/folks2,renatofilho/folks,renatofilho/folks2,renatofilho/folks2