New File Wizard - final version (hopefully)

Thanks for the review in the last PR, I reworked the wizard a little bit (last 6 commits are new, some are rebased/removed). Main points:

- Project selection is replaced by source folder selection
- Files are created by the wizard and no longer by the file creator
- Old wizards do now coexist with the new one. There exists only a single entry for the new one, called "New Scala File"

The empty file creator is still broken for some cases, but it can be resolved after M3 together with all other outstanding issues. If I have not overseen something critical this PR should now be ready for M3.
I noticed right now that this condition makes no sense for src folders in subfolders. I think it is safe to remove it altogether.