Bug 911055 - [Dialer]DuT doesn allow to dial a <<pause>>

Yeah, I really didn't want to modify dtmf_tone.js because I thought it may be confusing since that library is meant to play and to send single tones ;) On the other hand, since the tones are really being sent by the backend and they can be a group of tones, we are not currently playing anything on the device sending the tones :)

So after my previous paragraph, would you be in favor of moving all the new functionality currently included in telephony_helper.js (I mean getNumberAsDtmfToneGroups(), playDtmfToneGroups(), playDtmfToneGroup() and playPause()) to dtmf_tone.js's prototype? Obviously we only need to publicly expose getNumberAsDtmfToneGroups() and playDtmfToneGroups() :) Or would you be in favor of including a new library dtmf_tone_groups.js including this functionality? Definitely it would ease its reusability ;)