added decoder_command_finished() to decoder_api.h

Some decoder commands are implemented in the decoder plugins, thus
they need to have an API call to signal that their current command has
been finished.  Let them use the new decoder_command_finished()
instead of the internal dc_command_finished().

src/decoder_api.h

@@ -105,6 +105,13 @@ void decoder_initialized(struct decoder * decoder,
 
 enum decoder_command decoder_get_command(struct decoder * decoder);
 
+/**
+ * Called by the decoder when it has performed the requested command
+ * (dc->command).  This function resets dc->command and wakes up the
+ * player thread.
+ */
+void decoder_command_finished(struct decoder * decoder);
+
 /**
  * This function is called by the decoder plugin when it has
  * successfully decoded block of input data.