Simplify background parsing/typechecking

This avoids a lot of needless reparses. All these reparses make the navigation extension nervous. Prior to this change, the nav bar would typically change three times when switching document.
It seems Parse() is designed to be long running, and it won't lock the UI. So there's no need to do this trickery with requesting reparses.
Upgrade to FSC 0.0.44
Fixes #452.
Fix subtle bug in Resolver provider there caret line was used rather than offset.
Revert ALT RETURN shortcut as it conflicts with the already defined global shortcut for quick fix.
Fix #302: Drag and drop for linked files
updated fsi icon
Added LanguageName element F# so it doesn't confuse the C# developers.