Open Documents Tracker -> FSharp.Editing

[cloudRoutine]

In order to move the functionality of some of the features over to VFPT.Core we need an open documents tracker on the Core side that will store the dirty buffers of the documents currently open in the editor. This will not remove the OpenDocumentsTracker from the Logic side, but will shift it's purpose to sending Document Open, Closed, and Changed events as well as the text of these documents to the Core side. It may need to apply changes to open docs received back from the Core side, but it's currently unclear if that will be necessary.

In VFPT.Core the OpenDocument record will store the text and the range equivalent to the snapshot span instead of ITextDocument and ITextSnapshot

The types in logic should be renamed to VSOpenDocument and VSOpenDocumentsTracker

Code Located @
https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharp.Editing.VisualStudio/ProjectSystem/OpenDocumentsTracker.fs