Shorten the needed sources from
calls_sources, calls_enum_sources, calls_resources,
wl_proto_sources, wayland_sources,
to
calls_sources
so adding new things such as generated dbus sources doesn't have to
happen in four locations but just one.
* 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