Fix modem addition/removal and deal with ModemManager appearing/vanishing
Modems being added or removed were not working. To fix this, we pay attention to the "object-removed" event and not just "interface-removed".
Also, to deal with ModemManager appearing and vanishing, we add a GDBus watch on ModemManager's D-Bus object.
Finally, we provide appropriate UI feedback when it's not possible to make a call.