This class allows us to display a legend for categorical data.
vtkHeatmapItem has been updated to use this new class.

Other changes made in this commit include:

* Improved appearance and behavior when a legend has been scaled by
  a vtkContextTransform

* An option to draw a border around the vtkColorLegend.

* vtkColorLegend now obeys vertical and horizontal alignment unless
  SetPosition() was called.

Change-Id: I6c7c782c6f3da78103b893c43d068e81e9ef78a7