Unification of CMake build between gui and core

In https://gitlab.eclipse.org/eclipse/simopenpass/simopenpass/-/merge_requests/15 and https://gitlab.eclipse.org/eclipse/simopenpass/simopenpass/-/merge_requests/16 a lot has happened that improved the build and install step of the gui. However, one could think about unifying the CMake build structure between gui and core and by e.g. extending internally defined functions like cmake/HelperMacros.cmake so that they also handle the build and install of the gui.