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.
How does Java deal with interfaces?  I feel having the interface defined right with the Interaction will clutter the file.  That said, this location does seem apt for this interface.