Adds a generalized version of the 3D-Renderer LoadingDialog to improve
UX by showing that CityDoctor is currently busy doing something in the
background of indeterminate length, e.g. opening a file or waiting for
the Schematron thread to finish.

Adds the CppTest marker-interface to exclude Healer tests that  were
interfacing with CGAL from the build. These tests are needed for the
development of the replacement library, but shall not cause build
failures in the meantime.