Fixed slider event regression in 3.10.0

Fixed regression introduced in 3.10.0, with stale facade properties sticking around in the case of "no subscribers".

Added unit tests for both the subscriber and no subscriber case.

All unit tests pass:
✔ [Total]: Passed: 10859 Failed: 0 Total: 10954 (ignored 95)

Slider functional tests pass.
Slider examples work manually.

This impacted slider's thumbMove defaultFn which was using a "source" property
added to the facade to figure out whether to set the value or not, and
commonly had no listeners (only surfaced by manual testing, which was why
it got missed - unit and functional tests all passed).