issue 1501 and various improvements on unzipping code, importing

progress messages, fixes on import media bugs

A general close inspection of the import code. There were multiple bugs on the media import (written earlier by me for 2.0.1, but never really tested, he, he), should be working better now, importing files when needed, skipping if the file already exists in collection.media folder and looks the same (only the first 1024 bytes are compared for speed reasons and of course only when filenames match).

Much better feedback to the user, as this operation can take a while.