What:
A very very rough W.I.P. to start some initial discussions about a "virtualized" PF4 table.

fixes #1229

Some things I've attempted so far:

• pull in some of the Reactabular virtualized components and wrap our existing table Body, BodyWrapper, and RowWrapper components with virtualized extensions (accepting those as props in our current Table)
• Render a simple virtualized table with percent widths. In order to scroll a virtualized table, the author recommends using flexbox for flexible widths with virtually scrolled rows. Needs more discussion and some CSS review, but I've included some sample CSS to tinker with:
  https://reactabular.js.org/#/features/virtualization?a=using-relative-column-widths
• Render a sortable virtualized table. It's important to keep in mind that virtualized rows are dependent on a scroll position (in order to performantly reuse them). For now, I have made the simple assumption that after virtualized sorting occurs to scrollTop, but we can certainly account for an existing scroll position if desired. The scrollTo method should allow the consumer to scroll to any desired row programmatically.

Still working on:

• provide an AutoSizer (similar to react-virtualized Autosizer). These essentially just wrap the table and adjust the height/widths appropriately on browser resize. This should not be needed. PF4 tables already responsively size the row widths and the virtualized table resizes for this.
• provide an option and an example to override the scroll container. This allows the consumer to define the scrollable surface. Similar to WindowScroller from react-virtualized or simply using the existing functionality in reactabular-virtualized. After the two options above are finished, we can support a very similar API to the one in use today:

<WindowScroller scrollElement={document.getElementById('content-scrollable')}>
        {({height, isScrolling, registerChild, onChildScroll, scrollTop}) =>
            <Table
              caption="Sortable Virtualized Table"
              cells={columns}
              rows={rows}
              bodyWrapper={VirtualizedBodyWrapper}
              rowWrapper={VirtualizedRowWrapper}
              sortBy={sortBy}
              onSort={this.onSort}
            >
              <TableHeader />
              <VirtualizedBody height={height} width={width} onScroll={onScroll} rowKey="id" tableBody={this.tableBody} />
            </Table>}
      </WindowScroller>

• test out other Table variations and provide examples w/ virtualized scrolling (selectable table, actions table, collapsible table, etc). These should all work similarly to the way they do today. Selectable table and actions table have been implemented. Collapsible is currently out of scope for this because dynamic row heights in conjunction with virtualized rows introduces problems we should probably avoid for now.
• Continue enhancing the Reactabular API surface with latest React methods and propose those changes upstream in Reactabular. In this PR, I have started these enhancements (using new Context API, use refs more appropriately, start to upgrade the lifecycles methods to React 16.4 plus. Opened 1628 for this.
• closes PF4 Bring Reactabular in-house #1628 bring reactabular-table source in-house. This is necessary to continue improving the base table used and to remove legacy React warnings caused by the old ref interface there.
• rebase PF4 React gatsby updates
• tests
• implement scrollable data grid a11y as best as possible (Example 3).
• convert to typescript where possible

Running demo:
http://pf4-table-virtualized.surge.sh/virtual-scroll/virtualized/

Additional issues:

cc: @karelhala @suomiy