File checker rework: a) bugs fixed, b) more robust detection, c) many tests added.

[TobiasKnauss]

Many tests added, especially for TAR archives, because the old FileChecker was faulty, because it detected many TAR archives as something else. The FileChecker class has been reworked completely to achieve a correct and more robust archive format detection.

bugfix: SevenZipSharp fails to unpack certain tar archives. https://stackoverflow.com/questions/53323255/sevenzipsharp-fails-to-unpack-certain-tar-archives

[squid-box]

It's been a while since I actively worked on this, sorry about that.

I like the overall gist of the PR, but given the changes that have been introduced since it was created it's not as straight forward as just accepting it. I've made a note to go through this and try to align these changes with the current state of my code base - but I can't promise a date when it's done.

If you have time / feel like resolving the merge conflicts I'll include it, otherwise I'll resolve them at some point and include this then.