Add filechooser unittests.

It creates files with names using unciode, bytes, as well has some existing unicode files in a zip.
It makes filechooser browse to that directory and then tests that the unicode filenames are present in filechooser.files when filechooser.path was set to unicode and tests that the bytes filenames are present in filechooser.files when filechooser.path was set to bytes.