ProjectProvider that piggybacks on F# project system

This PR contains implementation of ProjectProvider that fetches compilation flags\source files from native F# project system. I hate the very idea of reflective calls but it seems that this is the simplest solution comparing to the work that should be done to fully replicate logic used by project system. One of tie breaking examples is including second order dependencies to the list of references: when desktop project references portable library then MSBuild appends to the regular list of references a bunch of facades that forward portable types to the regular ones. This data cannot be obtained without deep integration with MSBuild.

Since now we just read values from the project object - caching is not necessary so conceptually ProjectProvider becomes just a proxy.
LanguageService stayed the only consumer of SolutionEvents so these two classes were merged.

This PR should fix #91