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.

Changes the ImplicitGeomeryHolder class to accept Visitors on itself
instead of deferring the call to the prototype geometry. This fixes the
occurence of some CCM errors during checking and is required for the
implementation of the planned topological checks.

fix solid self intersection incorrectly ignoring degenerated triangles depending on planarity settings

Adds the handling and creation of the embedded database in RAM, which is
used for the in-memory and fallback modes.

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.