XR input sources interact with Elements

[Maksims]

Allows XR (VR and AR) Input Sources to interact with Element components. This simplifies creating UI for XR.
Events: hoverstart, hoverend, pressedstart, pressedend, click will now be fired when interacting with UI using input sources: gaze, screen (AR), handheld, etc.

New APIs:

pc.ElementInput#options.useXr // new property for constructor options
entity.button.on('selectstart', function(evt) { }); // fired when input source primary action is started on UI element
entity.button.on('selectend', function(evt) { }); // fired when input source primary action is ended on UI element
entity.button.on('selectenter', function(evt) { }); // fired when input source starts to hover over UI element
entity.button.on('selectleave', function(evt) { }); // fired when input source is not hovering over UI element anymore
pc.ElementSelectEvent // new event object to handle input sources interactions with Elements
pc.XrInputSource#id // unique number to identify input source. It will not persist for physical hardware reconnections.
pc.XrInputSource#elementInput // boolean to toggle if input source should interact with Elements. By default is True.
pc.XrInputSource#elementEntity // null or Entity over which input source ray is hovering

I confirm I have signed the Contributor License Agreement.

[Maksims]

This PR will have to be updated after #1991 is merged.

[willeastcott]

OK @Maksims, #1991 has now been merged. 😄

[Maksims]

Updated the PR, so now it uses updated input source transform methods.
Ready for merging.