protocol-native: Add signal receiving capability

This patch extends the client subscription API, so that signals sent from
PulseAudio can be processed. Within PulseAudio, a signal can be emitted
using pa_signal_post(). The interface can be used to notify the client of
events that are not covered by the subscription API (for example a button
press event on a bluetooth headset).

Setting up signal notification is very similar to using subscriptions.
First the client needs to subscribe with pa_context_subscribe_signals()
and then sets up a signal handler using pa_context_set_signal_callback().

The signal handler will receive three arguments in addition to the usual
context and userdata:
object_path - string that specifies the origin of the signal
signal - string that specifies the type of the signal
signal_parameters - optional string for additional information

src/pulsecore/core.h

@@ -144,6 +144,7 @@ typedef enum pa_core_hook {
     PA_CORE_HOOK_SAMPLE_CACHE_NEW,
     PA_CORE_HOOK_SAMPLE_CACHE_CHANGED,
     PA_CORE_HOOK_SAMPLE_CACHE_UNLINK,
+    PA_CORE_HOOK_SEND_SIGNAL,
     PA_CORE_HOOK_MAX
 } pa_core_hook_t;