Auto detect observed fields on fill

Currently hooked up to fills for str and Calendar (locators that are usually form inputs).
Not hooked up to fill ObservedText elements, of course.
Using observed field autodetection instead of ObservedText

I didn't get rid of ObservedText outright, since I can easily imagine
cases where the an observed field isn't marked as such in the UI and we'll
need to manually mark it.
Merge pull request #445 from seandst/auto-observe-text

Auto detect observed fields on fill