- Using vtkHardwareSelector to capture the cell ids on the screen at any
   given time. This is reused as hover events occur.

 - Using vtkHoverWidget to receive timeout events when the mouse hasn't
   moved, and update the hover text then.

 - Added more functionality to vtkHardwareSelector to be able to query
   regions within the captured area (for drag selections) and being able
   to search out from a point to a given threshold distance for a cell
   (for hover text).

 - Added vertex and edge hovering to graph layout view, and edge hovering
   to tree area views.