Hide Dontprint icon in private browsing mode

Dontprint makes little sense in private browsing mode because
one needs to sign in with Google to use it.
Simplify redirect page.
Different article for detect-screenshot
Links to website on welcome page.
Fix trivial bug (typo) in cropMargins()
Package version 0.2beta1

  * Add a build script that creates an xpi-package.
  * Resolve some legacy TODOs.