Allow to pass a ref for the underlying div

[veej]

Description

At the moment it's impossible to pass a ref to the underlying div element.
We should add an innerRef prop to FlexView and forward it as ref for the root div.

[giogonzo]

slightly different proposal: use forwardRef and update the ref typings as appropriate to refer to the underlying <div />. Rationale: no need to have a reference to the View element itself (no API exposed as instance methods), while you often need a ref to the underlying div to do dirty DOM stuff

[giogonzo]

Even better: since FlexView has no "public API" on the class/component instance, no need to expose the ref to FlexView at all. Exposing the one to the underlying div is more than enough, as in:

type FlexViewProps = {
  // ... more props
  ref: React.Ref<HTMLDivElement>
}