Folder organization support

I'm pretty new to both F# and VS SDK, so I'd really appreciate someone more experience to review and test this PR.

**Included features:**
- New Folder *- works on project level and existing folder*
- New Folder Above *- works on folders and files*
- New Folder Below *- works on folders and files*
- Rename Folder *- works on folders, it's necessary because standard folder renaming (F2) does change folder name, but does not update project file*

*Move Up* and *Move Down* work just fine with folders already.

**How it works?**
Unfortunately, using `Project.ProjectItems.AddFolder` does not work (new folder shows up in Solution Explorer, but project file is not updated), so I manually parse and update project file to add/rename folders. Because of that, every time VS shows *project file changed* dialog, and user has to click *reload project* button. But it's probably better than nothing.

**What's missing?**
- There is no way to move existing files to folders other than manually edit project file. There is already a feature request for that: https://github.com/fsprojects/VisualFSharpPowerTools/issues/168 but with folders it makes much more sense