Add new dbus helper functions

Parsers to ease handling of messages with containers.

And message handler registration that is easy to unregister too.
Improve oFono voice call state tracking

Tracks all voice calls on all modems.

Emergency call state taken from both modem and voice call properties.

Re-scans modems and voice calls on mce startup and if ofono restart
is detected.

[mce] Improve oFono voice call state tracking
Merge pull request #98 from spiiroin/callstate_ofono

Improve oFono voice call state tracking