Drop 3rdparty submodule

:construction: DON'T MERGE THIS!!!

Proof-of-concept

* drop all test folders
* drop all .git*, travis, phpunit, scrutinizer, etc files
* just leave LICENSE as plaintext
* compared to old 3rdparty repo: nothing changed in source (except some comments)

Notable part of this PR is the first commit and there the improved .gitignore.