Cleaned up `CMakeLists.txt` files in subdirectories so that the minimum reqiured
version is set only at the beginning of the top-level `CMakeLists.txt` file

Not needed right now, the build problem was caused by Geant4 only

A couple of changes have been made here:

- The MuonPlugin needs GeoModelXML (it's the de-facto a GMXPlugin), but
  the build of GeoModelXML was triggered when the PLUGINS were built.
  So, now the option `GEOMODELATLAS_BUILD_PLUGINS` also set the option
  `GEOMODELATLAS_BUILD_GEOMODELXML` to True.

- It was not possible to compile MuonPlugin alone, as the other Plugins.
  So, this also extend the MuonPlugin's CMake setup, for the plugin
  to be compiled standalone (i.e., without all the other plugins)

- This also conforms the CMake setup of MuonPlugin to the setup of all
  the other plugins in `GeoModelPlugins`.