Added Interaction.Hover.

Interaction.Hover acts on any Component that has implemented the
Hoverable interface, which allows the Interaction.Hover to query
the Hoverable for data given a cursor position.

Added a hoverMode() call to AbstractBarPlot, which will allow
Interaction.Hover to replace the Interaction.BarHover.
The variable is called `lastHoverData` because internally it stores the result of the last `_doHover()`. I think the current name is fine.