request: have a new job replace an old job

[bendichter]

These new dash features look great, and are similar so something we need for our project. Essentially, we are running plots with multiple controllable parameters where a single parameter tweak might take a long time to render. If a user makes one tweak and then another, they will have to wait for the first plot to render and then wait for the second plot. We would like to change this so the second tweak cancels the current job and only runs the new job. This looks very similar to your cancel button, but it would be ideal if a number of widgets could be part of a group where they all cancel each other because they are all parameters of the same plot.