core: add simple message interface

This patch adds a new feature to the core which allows to send messages
to objects. An object can register/unregister a message handler with
pa_message_handler_{register, unregister}() while a message can be sent
to the handler using the pa_message_handler_send_message() function.
A message has 4 arguments (apart from passing the core):

object_path: The path identifying the object that will receive the message
message: message command
message_parameters: A string containing additional parameters
response: Pointer to a response string that will be filled by the
          message handler. The caller is responsible to free the string.

The patch is a precondition for the following patches that allow clients
to send messages to pulseaudio objects.

There is no restriction on object names, except that an object path
always starts with a "/". The intention is to use a path-like syntax,
for example /core/sink_1 for a sink or /name/instances/index for modules.
The exact naming convention still needs to be agreed.

src/pulsecore/meson.build

@@ -20,6 +20,7 @@ libpulsecore_sources = [
   'filter/lfe-filter.c',
   'hook-list.c',
   'ltdl-helper.c',
+  'message-handler.c',
   'mix.c',
   'modargs.c',
   'modinfo.c',
@@ -75,6 +76,7 @@ libpulsecore_headers = [
   'filter/lfe-filter.h',
   'hook-list.h',
   'ltdl-helper.h',
+  'message-handler.h',
   'mix.h',
   'modargs.h',
   'modinfo.h',