Zopfli in prefs

New text on metadata checkboxes
Translated Zopfli prefs
Moved file-related methods to base Worker
Made order of operations opportunistic rather than strict when queue is small

Refactored queueing of tasks a little
Process smallest files last for better endgame
Filetype-dependent definition of "large" files
Delete EXIF regardless of file size

It was a quirky hack that made preferences misleading
Ensure temp files are cleaned up after save error