Added an help button in the menu that displays a guided tour of the UI [WIP]

This is a work in progress (especially the design/content parts) but it can already be tested.

I will be adding this feature for my users and was wondering if it could be of interest to anyone else.

Any input is welcome :)
These l10n strings need to be added to the default (i.e. `en-US` locale) as well: https://github.com/mozilla/pdf.js/blob/master/l10n/en-US/viewer.properties.