Extending the "Tour" to allow extensions to use the feature

As mentioned in our meeting earlier today, it would be really nice for extension developers to be able to use the same help popup mechanism as the beta Tour feature uses to introduce the extension once its loaded into the workspace.

Having access to an API to indicate where an extension has been loaded into the toolbox and have it highlighted for the user would be really nice; plus maybe the option to indicate one or more blocks within an extension to highlight functionality.

Perhaps some sensible limit on the number of these that can be shown for a given extension would be good too; to prevent excessive pop-ups when users add an extension (and to inhibit any kind of malicious extension that would pop-up forever)

Personally, I think a nice workflow would be to offer an extension the option to show an initial, single pop-up to indicate where it has been loaded into the toolbox, then have a button on this initial one to continue the tour into the extension where additional blocks could be highlighted, or a second button to just stop the tour at this point.

I've included a super-quick mock-up of the intro popup below (pretending that the loop blocks are an extension)

(Pinging @abchatra for reference)

Start by locating the beta Tour implementation and the extension-loading and toolbox entry points. Define how an extension would request an initial toolbox highlight, optional block highlights, and continuation or dismissal controls, while enforcing a sensible display limit. Done means the API and popup workflow are specified and usable by extension developers.