Partition: add a local idle_monitor

Make idle events per-partition, but leave Instance::EmitIdle() and its
underlying idle_monitor which broadcasts idle events to all
partitions.

src/Instance.cxx

@@ -180,10 +180,7 @@ Instance::OnRemoteTag(const char *uri, const Tag &tag) noexcept
 void
 Instance::OnIdle(unsigned flags) noexcept
 {
-	/* send "idle" notifications to all subscribed
-	   clients */
-	client_list->IdleAdd(flags);
-
-	if (flags & (IDLE_PLAYLIST|IDLE_PLAYER|IDLE_MIXER|IDLE_OUTPUT))
-		OnStateModified();
+	/* broadcast to all partitions */
+	for (auto &partition : partitions)
+		partition.EmitIdle(flags);
 }