Use incremental lexing from FSharp.LanguageService

As per this comment https://github.com/fsprojects/VisualFSharpPowerTools/issues/108#issuecomment-36895027.

Incremental lexing will give better performance than lexing the document again after each 200 ms.
I tested it with FCS project; it seems a bit snappier.

The use of FSharp.LanguageService is particularly important because it helps us to migrate from half-broken ProjectParser to a proper F# project parser via reflection.