==== Requirements ====
* Must appear quickly for immediate feedback of Activity icon click/touch. No more than 1000ms, ideally some visual update should happen
in 100ms (e.g. white blanking before we currently see the activity pulsing icon draw). Some UI content, such as image thumbnails, can be delayed as long as the rest of the UI & intermediate thumb placeholders are there.
* Must allow closing/stopping/cancelling so the user can back out quickly if they change their mind
* Should cope with the user using the Frame or alt-tab to switch elsewhere without completing the dialogue, and possibly returning later