Alternate extraction interface

Some unpackers cannot unpack files in ProcessFile. They need to unpack in CloseArchive. This unpackers mostly have a single unpack function for a single archive. This has several downsides like no support for text search. There is also a performance problem as the files listed in ProcessFile have to be added to the single unpack function of the unpacker.

The idea is to introduce a new API function 'UnpackFiles'.