Migrate to new "Australis" design introduced in Firefox 29

* Add grayscale button for the new "Australis" menu
  introduced in Firefox 29. The button opens a subview
  with the same menu options as the original panel button.

* Remove the legacy Dontprint toolbar button to reduce code
  complexity. This means that users on the Firefox ESR
  channel will now have neither a Dontprint toolbar button
  nor a panel button. But we still provide the Dontprint-
  submenu in the "Tools" menu, so the toolbar button is not
  essential anyway. Also, the current ESR will run out in
  October 2014 and the next ESR will be based on the new
  Australis design.

* Remove implementation and usage of
  Dontprint.getToolsMenuLabel() (introduced in commits
  ca39896e and d3a91129).

Fixes #11.