Fix some VS IDE performance issues.

- cache scan results so it can be shared between different instances of
  DafnyTokenTagger
- Instead of rescanning the whole text buffer upon a text change, only rescan
  the text span that the text change is in.
- set the timer to half a second to match the comment at the beginnig of
  the file. The event change are only passed along to Dafny when the user stop
  typing for half a second.
- Change framework dependence so the project can work with version 4.5 and
  higher.

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