Stash SVG points selections for faster pan

[etpinard]

PR #2527 introduced a 'draggedPts' trace category (see #2527 (comment)) which was later deemed a potential overkill by @alexcjohnson in #2527 (comment) to make the pan hot code path avoid large .selectAll() (which can be very slow).

What we should try to do instead is: stash those points selections (probably in fullLayout._plots[subplotId]) early (probably during the trace plot step) and use them directly in pan (and scroll?) hot code path.

[etpinard]

While doing this, we should investigate ways to remove more things from that hot pan code path like various fullLayout._has return values (mentioned here).

Moreover, we could perhaps improve performance even more by combining Drawing.setScale and Drawing.setTranslate into one method (ref).