Fix the "Never suspend tabs that contain unsaved form inputs" feature

[RJVB]

There were several issues with this feature:
* the `event` variable isn't defined
* Firefox had unclear support for the `keyCode` property, which also
  has been deprecated for some time now.
* The key range seemed to consider only characters from '0' to 'Z',
  excluding lower case common text for instance.

With this patch the feature works, as shown by the screenshot:

[RJVB]

@dvalter: It would be really nice if you could integrate this change quickly, as I can only run a Firefox version on my main machine that doesn't except unsigned extensions (and I'd rather not deal with changing the extension ID given how large my session is)!