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.
Maybe change the function name to like missingItemsFrom or similar.  The problem I have is that one might expect A diff B == B diff A which is not the case here.