audio_output: added method pause()

pause() puts the audio output into pause mode: if supported, it may
perform a special action, which keeps the device open, but does not
play anything.  Output plugins like "shout" might want to play silence
during pause, so their clients won't be disconnected.  Plugins which
do not support pausing will simply be closed, and have to be reopened
when unpaused.

This pach includes an implementation for the shout plugin, which
sends silence chunks.

src/output_api.h

@@ -81,6 +81,16 @@ struct audio_output_plugin {
 	 */
 	int (*play)(void *data, const char *playChunk, size_t size);
 
+	/**
+	 * Pause the device.  If supported, it may perform a special
+	 * action, which keeps the device open, but does not play
+	 * anything.  Output plugins like "shout" might want to play
+	 * silence during pause, so their clients won't be
+	 * disconnected.  Plugins which do not support pausing will
+	 * simply be closed, and have to be reopened when unpaused.
+	 */
+	void (*pause)(void *data);
+
 	/**
 	 * Try to cancel data which may still be in the device's
 	 * buffers.
@@ -104,6 +114,7 @@ enum audio_output_command {
 	AO_COMMAND_OPEN,
 	AO_COMMAND_CLOSE,
 	AO_COMMAND_PLAY,
+	AO_COMMAND_PAUSE,
 	AO_COMMAND_CANCEL,
 	AO_COMMAND_SEND_TAG,
 	AO_COMMAND_KILL