Allow / display highlighting of live data in feature view.

[damellis]

That is, when showing the live data as features, it should be possible to select from the history of live data (while the pipeline is paused) to create new training samples. Currently, it seems as though highlighting is supported but not displayed. Or maybe the highlighting is being done by the invisible input data plot rather than the feature plot?

[damellis]

To do this, we should probably instantiate separate plot(s) for the live feature data, rather than simply grabbing pointers to the last stage of plots shown on the pipeline tab.