Collection corruption

[bflorat]

In some cases on big collection volume, several tracks
can get the same track id.

[bflorat]

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Fixed : we now take album into account when computing track
MD5. 

Note however that event with this fix, a design problem
could accur if two tracks with no name, no album, no author
have the same length and type. It can occur if option 'use
directory as album name' is unchecked and collection has
very few id3 tags. This will not be fixed because it is
design. Note that in the case of a very poorely tagged
collection, logical perspective will probably not be used at
all because it is useless.

Possible problem : low : 2 physical files totally different
could be asociated to the same track. Then stats could
erronus and a property set on a track could be seen for a
wrong file.

[bflorat]

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Followup:

Added a fix to definitively prevent from the very unlikely
bug described before ( track confusion for two files having
none id3 tag , same name, same length and same size ) :
Added a low level checksum on files by reading 10 bytes
inside the file itself. This sub-checksum is token into
account in the file checksum. ( see Util.getFileChecksum )

Conclusion : This phantom bug is definively fixed