addKeyListener utility makes overrides difficult in TreeWidget

[colin-grant-work]

Bug Description:

It seems that the native KeyboardEvent listeners attached by the addKeyListener utility always fire before listeners attached by React, even if the React listeners occur on a child element and call event.stopPropagation(). That is bad enough. However, addKeyListener itself will call .preventDefault() and .stopPropagation() if the return value of the handler is not false. That means that the listener attached to the child on the React side may never be called if the native event handler returns a Promise for example.

Steps to Reproduce:

1. Create a TreeWidget whose node includes a focusable element with a keyDown handler that calls .stopPropagation().
2. Focus that element and start striking keys.
3. Observe that your listener is never called for space, enter, escape, left, right, up, or down, but the default tree behavior does occur.

Additional Information

• Operating System:
• Theia Version: af11fa0