Ajax attached to Views exposed filter form does not trigger callbacks

Problem/Motivation

If you try to use #ajax on a Views exposed filter form element, the callback is never invoked.

Steps to reproduce

1. Attach #ajax in a views exposed form
2. Expect the callback to be trigger as if attached elsehwere
3. No callback is triggered

Proposed resolution

Handle the mix of GET from the View and POST from the callback by determining if the triggering element is not the exposed form submission, but is instead the element with the attached ajax callback.

Remaining tasks

Fix the issue described in #155

User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

Ajax callbacks can now be attached to elements inside a Views Exposed Form.