Fix new folder behaviour

Fix #309.

Now we hide `Add --> New Folder` menu item and create physical folder when invoking `F# Power Tools --> New Folder`.

The only issue left is that empty folders will be disappeared after the project is reopened. I guess it is the limitation of `ProjectItem.AddFolder`. The method above should create `<Folder Include ="FolderName"/>` node in case of empty folders, but it doesn't appear to do so.