Add a progress bar dialog with a cancel button

[mhucka]

Currently, you can't cancel an ongoing lookup. You can only stop it by refreshing the browser page, but that's not obvious. Better would be to have a typical progress bar dialog and add a cancel button that the user could click to stop things deliberately.

[mhucka]

The following resources and examples might be useful when implementing this:

• https://stackoverflow.com/questions/10651493/google-app-scripting-progress-bar
• https://stackoverflow.com/questions/72867516/bootstrap-progress-bar-with-apps-script-variable
• https://stackoverflow.com/questions/67605557/how-to-get-continuous-output-progress-bar-in-google-script
• https://stackoverflow.com/questions/71850825/google-docs-apps-script-loading-indicator
• https://stackoverflow.com/questions/35343688/google-apps-script-file-upload-progress-indicator
• https://groups.google.com/g/google-apps-script-community/c/jg9NVSk4LlQ

[mhucka]

With the recent rewrite of Boffo to make it extremely fast, this feature is no longer a priority. I'm downgrading it.