Progress bar for all reactive elements

[billdenney]

This is related to (but distinct from) #609

I have an app where I would like to have a top-level progress bar that shows progress on all reactive elements that are to be completed. I think that the ideal scenario would look at all of the reactive elements as a directed graph and count all reactive elements that will update (tracking cycles only once) and then use that as the denominator for the progress bar.

Then, as reactive elements are executed, they are counted. If it is the first time that they are executed, they move the counter progress up by that amount. If it is after the first time they are executed, they progress the counter by that amount and add one to the total count until completion (so, it can go backward or appear to stall).

The default notification would be some form of indicator of the reactive element that is being run and the count of times it has run.

It seems like it would need to run within the loop that handles reactive elements (above the typical server functions), but if there were a hook into that loop, it seems like it could be easy to create. (I looked for said hook and didn't find it, but I'm not a shiny expert.)