[RFP, Autofill] Switching dialog field identification to int.

[RFP, Autofill] Switching dialog field identification to int.

Upside:
 * It's clearer to see if we care about a field type or a field in
   a specifc section. (Because the latter requires a section spec)
 * It'll be easier to have section-specific calls on a section object.
 * Both Views and Cocoa will be able to look up fields via their tagging
   system - less code. (We _could_ tag with addresses, but it turns
   into a nasty-cast fest without much pointer validation)

Downside:
 * It's a big-ish change.

BUG=none

Closed, because it went nowhere - but gave a lot of insight into the code