Ability to "guess" the first part of a multipart RAR archive

[TranceLove]

Depends on #2096.

When user extracts a multipart RAR, Amaze should extract from the first part of archive, even when the user tries to extract from parts other than part 1.

Example of expected behaviour :

1. User tap on part 5 of a 20-part RAR, e.g. file.part05.rar
2. Amaze guess the first part of the multipart RAR to be file.part01.rar
3. RarExtractor work from file.part01.rar instead and extract files from there

Archive viewer (CompressedExplorerFragment) may not need this however, to be the same behaviour as desktop archivers like FileRoller or 7zip FM.