AdapterIO: Add IsRunning method

* The backend need to know if we can really get data.
* Adapt WaitForData to be more reliable using this method.

headers/private/media/AdapterIO.h

@@ -58,6 +58,8 @@ public:
 	virtual status_t				Open();
 	virtual void					Close();
 
+	virtual bool					IsRunning() const;
+
 			void					SeekCompleted();
 			status_t				SetBuffer(BPositionIO* buffer);