PreviewClass -> Previewer and remove gestures from it.

Also, I've fixed the identetion problems I had.

Reviewer and PreviewClass inheriting from an AbstractFlashcardViewer.
Hopefully it will fix the messy of PreviewClass.
 Issue 1508:        Typed answer appears at the top and has no comparison
 Showing a place holder instead of [[type]] location in question.
  showing user answer instead of [[type]] location in answer.

Hide keyboard when user want to type the answer