Use the "number" property of the dial pad for the dialed number which
takes care of cycling buttons. While we're at it, also make the clear
button external to the number entry box, enlarging it and turning it
into a backspace button.
Closes#12
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.
Closes#15Closes#16
