Add a document transform to insert index and target nodes just before
any CMake domain cross-reference node. This will make references to
CMake domain objects appear in the index. Also add a comment explaining
why it cannot be done in a result_nodes method of the CMakeXRefRole.
Add identifiers for variables, properties, policies and modules.
This will allow QtCreator to show relevant documentation if it learns
more about the context of the contents of cmake files.