Styleable scroll-area for @finos/perspective-viewer-datagrid

[texodus]

Reorganizes <perspective-datagrid> DOM structure thusly:

<perspective-viewer>
    <perspective-datagrid>
        <table></table>
    </perspective-datagrid>
</perspective-viewer>

.. with perspective-datagrid element itself acting as the scroll container. This means you should:

• Listen to scroll events from this element
• Call its scrollTop to move.
• Use it to access methods like get_tds().