Introduce new PhoneNumber class

The PhoneNumber class is useful as an interface type, so the caller isn't required to pass in a specific phone number format.

This should also reduce the number of parsing / serializing operations, since the phone number is stored in a parsed representation instead of a string.

This was not tested with a working modem yet. Since QML checks types only at runtime, it is possible I forgot to update some places, so please review carefully.