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.

Use a BufferedInputStream for counting the bytes of a ZipEntry to reduce
the number of read operations on the hard drive.

Added the getDisplayName() function, which returns the filename without
the zipfile subpath prefix

Adds supporting functions for stream-reading of CityGmlZipEntry's
compressed CityGml-file, to allow validation of zipped files without
mounting them into the memory.