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 thinking that the default value perhaps should be `true` instead. (This can easily be changed if/as needed on a case-by-case basis in custom implementations.)