Datagrid header click-to-sort and assorted improvements

[texodus]

Adds column header click-to-sort to @finos/perspective-viewer-datagrid. This implementation differs slightly from the existing @finos/perspective-viewer-hypergrid in that:

1. It is single-click rather than double-click.
2. It supports multiple sorts on the same column, for row-and-column sort.
3. Is style-able via CSS

A few other small improvements:

• Adds a tree-line renderer.
• Debugger & profiler driven improvements to the double-buffering renderer, which optimizes framerate even further and reduces draw-in effect of updating the DOM.
• Fixes @finos/perspective-viewer-datagrid to work nicely when switched out for another plugin.
• Fixes column autosizing miscalculation when row and column pivot levels change.

[zemeolotu]

Thanks for the PR. Looks good!