[Cocoa] Add handler for javascript prompt/input. (#1567)

* [Cocoa] Add handler for javascript prompt/input.

Cocoa was missing handler for calling prompt("...")

This adds the popup and input handling

* [Cocoa] Add edit menu to get cut/copy/paste/selectAll on submenus, alerts, etc.

The existing implementation in the keyDown_ handler only works on items in the main view and not in subviews/modals