Support Pointer Events

[marklundin]

This PR adds preliminary support for pointer events on <Entity/> components. It replicates standard dom-like event behaviour such as event bubbling and stopPropagation() via synthetic events. It also support click

<Entity onPointerDown={() => console.log("Parent triggered" }>
  <Entity onPointerDown={(e} => {console.log("pointer down"}; e.stopPropagation()} />
  <Entity onPointerDown={(e} => {console.log("pointer down"}; } />
</Entity>

Supported events:
onPointerDown
onPointerUp
onPointerOver
onPointerOut
onClick

[marklundin]

Added additional onPointerOver, onPointerOut and onClick events

[willeastcott]

Aren't generic attributes lower case instead of camel case? e.g.:

<div onclick="console.log('ouch')"></div>