captureKeys flag

[georgealways]

Adds a flag captureKeys to GUI's constructor. If this is false, GUI will not call stopPropagation() on key events that originate from within the GUI. The user must then manage event propagation manually using the following or similar.

const gui = new GUI( { captureKeys: false } );
window.addEventListener( 'keydown', e => { 
  if ( gui.contains( document.activeElement ) ) return;
  if ( e.code === 'Space' ) jump();
}

Writing tests for this was tricky. The browser shim needed to be expanded to simulate event propagation.

Open question: Right now, folders titles are treated differently than Controllers. They never capture key events (mostly because of #69). I'm wondering if we should keep this special treatment, or if the captureKeys flag would apply to both Controllers and folder titles in the same way.

Note to self: the base for this branch was title-as-button. Merge that first.