added decoder_initialized()

decoder_initialized() sets the state to DECODE_STATE_DECODE and wakes
up the player thread.  It is called by the decoder plugin after its
internal initialization is finished.  More arguments will be added
later to prevent direct accesses to the DecoderControl struct.

src/decoder_api.h

@@ -34,4 +34,10 @@
  */
 struct decoder;
 
+/**
+ * Notify the player thread that it has finished initialization and
+ * that it has read the song's meta data.
+ */
+void decoder_initialized(struct decoder * decoder);
+
 #endif