Rename the functions 'isValidStream' to 'isSupported'.

The name 'isValidStream' is a little misleading because it doesn't check if the stream is really valid. Additionally, 'isSupported' can be naturally overloaded.

taglib/mpc/mpcfile.cpp

@@ -79,7 +79,7 @@ public:
 // static members
 ////////////////////////////////////////////////////////////////////////////////
 
-bool MPC::File::isValidStream(IOStream *stream)
+bool MPC::File::isSupported(IOStream *stream)
 {
   // A newer MPC file has to start with "MPCK" or "MP+", but older files don't
   // have keys to do a quick check.

taglib/mpc/mpcfile.h

@@ -221,7 +221,7 @@ namespace TagLib {
        * \note This method is designed to do a quick check.  The result may
        * not necessarily be correct.
        */
-      static bool isValidStream(IOStream *stream);
+      static bool isSupported(IOStream *stream);
 
     private:
       File(const File &);