ITextView.SnapshotPointAtCaret helper extension member
expose VSLanguageService.MakePointInDocument to better encapsulate obtaining PointInDocument from VS editor
GoToDefinitionFilter:
factorized fallback continuation code (now needing it three times)
simplify making PointInDocument, although since we need SnapshotPoint (to be simplified later hopefully) I'm using two steps (get snapshotpoint, then get pointindocument)