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 :)
I'm unsure if we really want anything except the following lines in the default localization files:
```
help.title=Show Help
help_label=Help
```
One of the open questions regarding this PR, in my opinion, is how we are going to deal with the localization for this feature. Given that we're not using it in Firefox, it should not (again my personal opinion) be included in the default l10n files. We really need to come up with a reasonable strategy here; one that doesn't unnecessarily increase the size of Firefox and the workload for localizers.