Chart hover tooltip should allow user interaction

[ahundt]

This is a separate feature request to be able to pin, sort and save the summaries that appear when hovering over a chart in tensorboard as per discussion in #1013.

[nfelt]

Rewording this FR to better capture the basic idea, which is that instead of being just a transient tooltip that often extends off the screen, we should make that table something you can interact with - i.e. you can scroll onto/over it (when it's not all visible), extract data from it (via copy-paste at a minimum, or perhaps a download-as-CSV button), etc.