ENH: Extension Wizard: Substitute CMake variable reference if possible.

Created by: jcfr

In case the main CMakeLists.txt of an extension contain references to CMake variable, these will be expanded if possible.

For example, if the CMakeLists.txt contains the following:

  set(EXTENSION_DESCRIPTION "The ${PROJECT_NAME} module provides Slicer user with any easy way to export models into a KiwiViewer scene file.")

the generated description file will have the following description:

  description The SlicerToKiwiExporter module provides Slicer user with any easy way to export models into a KiwiViewer scene file.