RenameFolder - UI + Command

I'm quite busy lately and that's all I can contribute for now. I'm sorry I can't do more now.

- **UI** - the same dialog which was already there for *New Folder* command, with the same validation (folder name duplication within solution). Added `OriginalName` to show folder name as default textbox value when renaming (empty string for *New Folder* command)
- **Command** - *Rename Folder* under *F# Power Tools*, active only when single folder is selected within Solution Explorer. When used, *Rename Folder* dialog is shown.

Rename logic should be plugged on line 248. Right now there is just a mesage box with new folder name.