Delay refresh when task add/edit dialog visible

Modifies auto-refresh to postpone the call to `window.location.reload()`
if the DOM contains a div matching `div.workitem-dialog`. In this case,
we re-test every 5 seconds to see if the dialog has closed.

***

I think the extension should CTFO if the user is potentially
adding/updating a task.
Merge pull request #2 from matt-holden/safe-refresh

Delay refresh when task add/edit dialog visible