The bar itself (`<pdf-features-notification>`) ships from mozilla-central, so
the viewer only hosts it.
Finally the maximum number of preferences is raised to 60, to match the change
made in mozilla-central in bug 2056102, since this adds a 51st one.
Given that Maps are used a lot more these days, this helper function is now used very sparingly and inlining the necessary code seems reasonable.
*Note:* Once [this proposal](https://github.com/tc39/proposal-object-keys-length) makes it into Firefox, we should be able to simplify all `Object.keys(...).length` call-sites.
Given that the "PREFERENCE" kind is used e.g. to generate the preference-list for the Firefox PDF Viewer, those options need to be carefully validated.
With this patch we'll now check this unconditionally in development mode, during testing, and when creating the preferences in the gulpfile.
