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.
There is not much I can do beside from forcing it how it should work. I already wasted hours in trying to find out which Java code is responsible for adding these entries to the cache, but I was not successful so far. For the search field I wasted about ~3 days getting it to work. If you know a better way on how to do it, then I'll be the first in using it. In the meantime I have to go the way Eclipse shows me.