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.
One is added, the "New Scala File". Without this code it is not possible to ask for the new wizard through the quick access search field. The "New" menu works independently from the search field (and the perspective still needs to be reseted to get this menu entry).