Add button to close a Mapillary track

[waldyrious]

Upon first encountering Deriviste, I tried following the instructions that appear on the sidebar, but once I clicked on a Mapillary track, the instructions panel was replaced with the Mapillary content, and I could no longer return to the initial screen.

There should be a button to close the current Mapillary track so that one could return to the default view with the instructions.

[waldyrious]

More specifically, the <div id="introduction"> in index.html gets completely removed once a Mapillary track is loaded.

There should be a way to restore it. Perhaps it should just be hidden with display:none and shown again if the Mapillary track is closed.

[waldyrious]

Perhaps even better: there should be a way to have the instructions visible while the Mapillary track is loaded, so that one can follow them step-by-step.

Maybe having the instructions available as a floating (collapsible) panel that can be shown whenever the user needs it would be a nice way to implement this.

[cbeddow]

I think the way to go here would be to add an info or help icon that can re-open the guide info, or even that links to a readme here in the Github repo. I am going to look into this soon!