Get rid of dtmf tone_stop and allow using DTMF over DBus
It wasn't used by any plugin backend and helps getting rid of a lot of code.
Add a SendDtmf
Method to the DBus interface.
See #343 (moved) for the Calls side and World/Phosh/phosh#592 (closed) for the Phosh side
Edited by Evangelos Ribeiro Tzaras