Changed the export functionality of ZipEntryManager to exporting the
validation reports of validation entries, instead of exporting carbon
copies of the input Zip files' CityGML files.

Adds alerts to inform the user of Exceptions/Errors that occurred while
parsing a ZIP file, instead of silently failing and closing the
openFileDialog window.

Adds a HashMap to allow CityDoctorController to remember the Checker of
a CityDoctorModel, to allow switching of models with ZipEntryManager
without having to rerun the Checks when an already validated model is
loaded again.

Short-term meaning, that the memory is only kept for this session and
only till the next CityGML source is loaded (even if the same zip file
gets loaded again).