Allow files named *.mp4 files to be handled like *.m4a.

This pull request follows message in forum [gmusicbrowser ignores .m4a files without tags and all .mp4](http://forum.gmusicbrowser.org/index.php?topic=761.msg3557#msg3557) 

This pull requests solves part of the issue.

## Method

I figured out in source code that hashtable `%Alias_ext` is intended to handle some extensions like another, for example files named like `*.m4b` are already meant there to be handled just like `*.m4a`.

So I applied the very same pattern for mp4 wherever m4b was mentioned.

## Observed result

With this change, gmusicbrowser appears to indeed handle files named `*.mp4` like `*.mp4a`.

Some of my files are now recognized and can be played. 

## Request

Can you review and accept this pull request ? Thanks.

## Next step

I'll handle the second issue ("can't read tag") mentioned in the forum separately.

Regards,