Add constants for the different standard item types
Fix an issue where the tab worker was not installed on tabs already open when the app's panel was loaded

  for (var tab in tabs) {
  }

Did not iterate over the tabs that already existed as expected.
Resolve this and avoid loading a page script until we actually need
to by loading page scripts on-demand when the user triggers autofill.
Fix autofill of fields without a unique ID or name

Autofill requests identified fields by ID and name.
These are not guaranteed to be unique however and could change
between the fields request and the autofill request.

Allocate a key when the fields are fetched and use that in the autofill
request instead.