shellDBus: Accept connector string rather than monitor ID

NOTE: This change is intended for the stable branch too.

The monitor index is not something that gnome-shell and gnome-settings-daemon seem to be agreeing about. Using the connector string is a much more reliable method of identifying a specific screen and we are indeed using this already for monitor labling.

So switch over to use the connector rather than the monitor index. If a user tries to use the old API, then the OSD will simply show up on all monitors (which is the status quo currently anyway).

This is required to fix gnome-settings-daemon#401 (closed) properly.