Improve File delete performance

[jukzi]

Currently Project/Clean... takes much time to only delete the output. Hopefully that can be improved - somehow

[HannesWell]

I wonder why RemoveDirectory is called if the element to be deleted is actually a file?

In general if the JDK implementation is too generic and therefore too slow we could consider adding a native (JNA, later FFM based) implementation by adding a delete() method to the NativeHandler? Maybe we can avoid some of the native calls done in nio.

[jukzi]

i have made delete parallel and don't plan further improvements.