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
