Simplify notification of upload progress.
Fix trivial bug in cropMargins()
Allow dontprinting from Firefox's builtin PDF viewer and from download dialog

Introducing new job type "pdfurl". At the moment, Dontprint does not extract
any meta data for jobs with jobType==="pdfurl". In the future, meta data should
be extracted by running Zotero's translators on the last page in the history.

Jobs with type "pdfurl" can be initiated in 2 ways:

  * When a PDF document is displayed in Firefox's builtin PDF viewer, a Dontprint
    icon now appears. Clicking this icon will start a "pdfurl" job.

  * If Firefox is directed to a PDF file and shows a download dialog then
    this dialog now offers an option to process the PDF document with
    Dontprint. Note that the download dialog only appears if the user
    has selected the option "always ask" for files of MIME-type
    "application/pdf" in the Firefox preferences.
Make updating default screen settings possible.

If the user sets all screen settings (width, height, pixels per inch) to
the respective default values, Dontprint now writes the value -1 for all
three settings to the preferences. When reading these preferences, any value
of -1 will be replaced by the default value. This behaviour makes it easier
to update the default settings: Whenever a new version of Dontprint comes
out with changed default screen settings, all users who were using the default
settings before will implicitly be updated to the new default settings.
Users who set at least one of the three screen settings to a non-default
value will stay on their custom settings. This is likely to be the desired
behaviour most of the time.
Add error handling when dontprinting from an incompatible web page.

Also fixes a bug where job.tmpFiles was set too late which caused
tmp files being left on disk if the job was killed due to an error.