Add PhotosFacebookItem

Fixes: https://bugzilla.gnome.org/700451
item-manager: Create a FacebookItem when we get one

Fixes: https://bugzilla.gnome.org/700451
application: Start the Facebook miner

Fixes: https://bugzilla.gnome.org/700451
properties-dialog: Show the source for Facebook items

Fixes: https://bugzilla.gnome.org/700451