Search makeover

Attached is a *fully backward-compatible* makeover of the search API for 1.4.0+. Also included is an implementation in the Users module (which is, frankly, more complex that most other modules in this regard). 

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #634
| License       | MIT
| Doc PR        | included

I also considered an events-based method which I discarded because of the need for a base helper class and other considerations. This is intended to lay the groundwork for future (Core2) improvements (possibly as referenced in #634). It is **fully functional** and requires nothing further for 1.4.0 release (IMO). Enhancements of course are always possible.