common: Add Settings URI support
As specified by https://docs.google.com/document/d/1N0uqNtVXEFn3cLgNMeN75mP_dpMpCco-7uw5PKow-_Q/edit
Another implementation exists in Switchboard. This allows any application to open a specific pane without knowing the gnome-control-center binary name.
This also enhances the cross-distribution support as this is a new distribution-agnostic way to open a setting pane.