Encapsulate new identifiers before rename

We have this function to determine that string is a valid identifier https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharpVSPowerTools.Logic/VSUtils.fs#L67

Maybe it's better to try `isIdentifier newName`, then, if it failed, enclose the name into the double backticks? And we should remove this validation from `RenameDialog`.