WIP: Add jQuery-based event dispatcher as part of RFC386

[simonihmig]

Largely inspired by ember-native-dom-event-dispatcher.

[simonihmig]

@rwjblue rebased this, and should be ready for review. Although this is not really that urgent, as it will only come into effect once Ember 4.0 (canary) is released.

I tested this locally by changing this line (https://github.com/emberjs/ember-jquery/pull/31/files#diff-168726dbe96b3ce427e7fedce31bb0bcR4) to some earlier version, so that the EventDispatcher is actually used, and tests were passing. But the real tests will only be possible once we have an Ember version w/o jQuery...

[simonihmig]

Reminder to myself: #148 introduced some changed semantics, that the jQuery related Ember APIs (including EventDispatcher tackled in this PR) are only "backported" when the jQuery integration flag is explicitly set to true. This PR needs to accommodate this, i.e. not override the core EventDispatcher when integration is not enabled!