Add 'mute' and 'dial pad' buttons, drop the 'hold' button, change the
size of the central buttons, make them use icons and drop the now
useless group size.
The oFono DTMF support doesn't match our model; you can only send
tones to the network and not to a particular call. The telephony
standards dictate that only a single call can be active at a time so
we just get the call to emit a "tone" signal if the call is in the
active state and get the origin to listen for the signal.
Some implementations, like oFono, only allow fixed-length tones to be
sent to the network, not started and stopped at will. To account for
this, we make the tone_start member function nullable and add a new
function, calls_call_tone_stoppable, to determine whether there is a
stop function.
* Preliminary documentation of CallsCall and CallsMessageSource
* Documentation and cleaning up of CallsCallData
* Add data files and meson rules to create gtk-doc documentation
* Move some macros from util.h to calls-message-source.h
