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.
Bumped version to 2.2alpha45
@branch-specific
Merge pull request #305 from brunodea/reviewer-previewclass

Trying to fix the PreviewClass messiness.