Extend Event interfaces with Element target

[NN---]

Events handlers in several interfaces such as Window, Document and so on can be safely typed with 'target: Element' instead of of 'target: EventTarget'

Before:

interface Event { target: EventTarget; }
interface UIEvent extends Event { ... }
interface MouseEvent extends UIEvent { ... }

interface DocumentEventMap extends GlobalEventHandlersEventMap {
 "click": MouseEvent;
}

After:

interface Event<Target = EventTarget>{ target: Target; }
interface UIEvent<Target = EventTarget> extends Event<Target> {  }
interface MouseEvent<Target = EventTarget> extends UIEvent<Target> { }

interface DocumentEventMap extends GlobalEventHandlersEventMap {
 "click": MouseEvent<Element>
}

Expected behavior:
Type of event.target should be 'Element' in relevant event handlers.

Actual behavior:
Type is always EventTarget which requires type assertions.

[mhegazy]

Duplicate of #299

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.