This change makes it possible to map vtkStringArrays to colors using
indexed lookup mode. The vtkStringArrays may be associated with
points, cells, or neither (field data). A slew of changes were
required to give this capability:

- Changed signature of vtkScalarsToColors::MapScalars(...) and
  vtkDiscretizableColorTransferFunction::MapScalars(...)  to take a
  vtkAbstractArray instead of a vtkDataArray. This enables mapping
  non-vtkDataArray arrays, such as vtkStringArrays. These changes are
  backwards compatible because vtkAbstractArray is the parent class of
  vtkDataArray.

- Changed vtkLookupTable::MapScalarsThroughTable2(...) to handle
  vtkStringArrays.

- Changed vtkColorTransferFunction::MapScalarsThroughTable2(...) to handle
  vtkStringArrays.

- Added vtkAbstractMapper::GetAbstractScalars(...) that returns a data
  array as a vtkAbstractArray - needed to retrieve vtkStringArray
  scalars.

- Added some error reporting when unhandled array type is encountered
  in vtkScalarsToColors::MapScalarsThroughTable2.

- Enabled use of vtkStringArrays for coloring in the surface mappers.

- Added some tests for mapping vtkStringArrays with
  vtkColorTransferFunction and vtkDiscretizableColorTransferFunction.

- Added some tests for coloring objects by vtkStringArrays in cell data
  and point data.

- Added logic to vtkMapper and vtkPolyDataMapper2D that generates a
  reasonable default lookup table when a vtkStringArray or other
  non-numeric array is selected for coloring by scalars. This requires
  making the vtkRenderingCore module depend on vtkCommonColor. Added
  tests for this feature.

Change-Id: I91e2e451217139a7daabd7f4a63de0b8ff707ad4