Add support for Shader Replacements on the Geometry representation

This feature allows the user to customize the OpenGL shaders used by the Geometry Representation by providing some replacement strings for the shader produced by VTK. The replacements are provided through a Json string like this:

[
  {
    "type": "fragment",
    "original": "//VTK::Light::Impl",
    "replacement": "gl_FragData[0]=vec4(1.0,0.0,0.0,1.0);\n"
  },
  // other replacements can follow
]

In the UI, the property is represented with a custom widget that allows to load Json from a file and save its path as a preset which is saved in the SMSettings (thus in user's configuration file). It is possible to select a preset, remove it from the list and edit the content of the Json string in a text edit widget.

To avoid to much overloading of the UI, the feature must be enabled first using the UseShaderReplacements property of the representation.