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.