Add Refs to HTML widgets

[hamsbrar]

Description

This pull request is about adding Refs support in Rad's HTML widgets.

Providing 'a declarative way' for everything sounds great but hard to accomplish. There are number of cases where users simply want the framework to get out of their way. We already provide few escape hatches like RawMarkUp widget but sometimes users want to access DOM elements that are associated with HTML widgets. Rather than forcing users to go underneath the framework ( document.getElementById ), we should provide a much easy and predictable way i.e Refs.

This feature is similar to React's Refs but with few differences:

• Only callback Refs, no legacy string/createRef APIs.
• Only on HTML widgets(we want refs to be a feature of widget not framework)

Progress

Update: With the support for RenderEvents(v1.2.0), we can add Refs without adjusting anything in the core.

• Implement Refs in HTML base widget.
• Add ref property to all HTML widgets(only HTML widgets, for now).

Benchmark results

• Benchmark results: vs 1.0.1 | vs 1.1.0 - [old results]

Updated Benchmark results

• HTML Refs(final) vs 1.2.0 (1)
• HTML Refs(final) vs 1.2.0 (2)
• HTML Refs(final) vs 1.2.0 (3)

[hamsbrar]

There are couple of things that I think can be improved.

1. RenderElement can provide more granular control over its lifecycle.

2. With more control over lifecycle, we can possibly short-circuit dispose(s).

These two optimizations cannot be made once we merge this PR, at least not without introducing breaking changes so we're going to experiment few things in #19 before merging this.