Initial stab at future Ajax support

[melvyn-sopacua]

Proposed changes

Add a callback hook just before data is consulted, leveraging
the logic that is already there (irrelevant keys, minlength, etc).

This needs more work to deal with requests that typically take longer
than the time it takes for a user to type multiple keys, but the API
for the callback is very unlikely to change as all that logic can
be encapsulated in the callback itself.

I'm not well-versed in JS unit tests, but will add them if the proposed
implementation is accepted.

Contributes to #4086 .

Screenshots (if appropriate) or codepen:

A (working) AJAX implementation is in this branch.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jpic]

The API looks good but I have a question: should we trigger this instantly or should we defer like 100ms to give the user a chance to type another character before firing the event ?