Only emit gestures that are being listened for

[dfreedm]

This change calculates what gestures are being listening for in the event path, and saves that list per pointer.

[dfreedm]

@sorvell PTAL

[dfreedm]

Here's an illustration for the behavior change:

<div id="a" on-tap="{{tap}}">
  <div id="b" on-track="{{track}}"></div>
  <div id="c" on-hold="{{hold}}"></div>
</div>
<div id="d" on-hold="{{hold}}"></div>

Before #36, a "tap" on #d would emit a tap gesture, even though no ancestor of #d was listening for it. After #36, the list of gestures to emit calculates what gestures are being listened for in the ancestors of node that received the "down".

In this example, any interaction with #c will only make "hold" and "tap" gestures.

Important note: even with #36, a tap on #c will dispatch the tap to #c and #b, even though the only listener for tap is #a

[sorvell]

LGTM