Folder organization support

- **New Folder** - adds new folder with given name, name validation checks if there is already another folder with exact same name anywhere within the project (check [this](http://marcinjuraszek.com/2014/03/folders-in-f-projects-how-to-do-it-what-to-avoid.html) post)
- **Move Folder Up** / **Move Folder Down** - moves folder up/down within current parent (project or another folder)

More info: https://github.com/fsprojects/VisualFSharpPowerTools/issues/116