Fix bug in options onUnload()
Make Dontprint work on Windows

  * Add menu entry to the minified Firefox menu on Windows.
  * Workaround apparent bug that prefwindow.onunload does
    not seem to get fired on Windows.
Fix regression introduced in commit 6510832
Trivial: remove some unnecessary code.
Add toolbar button.
Simplify updateDontprintIconVisibility()

There are three different menu items labeled "Dontprint this page" which
have to be set enabled or disabled depending on the currently displayed
page: In "Tools --> Dontprint" on Linux & Mac, in the minified Windows
menu and in the menu associated to the toolbar button. Rather than trying
to set the disabled attribute of each of these whenever a new page is
loaded, this commit sets the disabled attribute lazily whenever any
of the menus is about to be displayed. This reduces the code that's
executed on each page load and also allows to reuse code.
Add option "autoShowProgress"; implement getToolsMenuLabel()

  * The progress page now has a checkbox at the bottom. When checked (default)
    the page will pop up every time the user starts a new Dontprint job. This
    improves feedback, especially for new users.

  * The method Dontprint.getToolsMenuLabel() returns the label of the menu
    which contains the "Dontprint" menu. This is usually "Firefox" on windows
    or a local translation of "Tools" on other platforms. The method is used
    by some chrome pages that contain instruction on how to use some of the
    functionality of Dontprint.
Show progress bar for sending.

Estimate sending progress based on the file size. Google
seems to need around 6 seconds per MB, according to tests
with files of various sizes from various machines with
various internet connections.