Moved uniforms and GPU shaders to new class vtkShaderProperty

This MR simplifies and generalizes the mechanism used for setting custom uniform variables and shader replacements in mappers. An instances of the new class (vtkShaderProperty) is stored in vtkProp. With the new class, the management of shader replacements is removed from individual mappers to avoid the current code duplication.

An example of the use of the new class can be found here. Volume rendering tests have also been updated to use the new class.