Add support for Zip64 extensions

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Add support for Zip64 extensions to allow Zip archives to :
- contain more than 65535 entries
- contain entries than are larger than 4GB
- be larger than 4GB as a whole

This will allow Zip64-enabled archives to be properly opened, and for archives created with muCommander to use Zip64 extensions when needed.

When creating (or modifying) an archive, muCommander should use Zip64 extensions only when they are absolutely necessary. Using Zip32 (spec version <4.5) whenever possible will maintain compatibility with older Zip unpackers that are not Zip64-aware.