Remvoed the pointless implementation of a virtual function.
Renamed MP4BoxJob::finished() to MP4BoxJob::readyForDeletion().
Calling it finished() again and letting jobPool act as a proxy for MainWindow.

Instead of having all MP4BoxJob instances hooked up directly to
MainWindow, let jobPool take care of it. This makes things simpler
overall.
Default way for reporting errors in MP4BoxJob.
New way of error reporting in MP4BoxChecker.
Fixing two typos.
Hooking up the new MP4BoxChecker with the UI.
Directly look for errors.
Removed unused signal.
Typo.
New way of error reporting in MP4Extractor.
New way of error/result reporting in MP4FileAnalyzer.
New way of error/result reporting in MP4Muxer.
We shouldn't need those.
Quotation marks in messages.
Make the progress of jobs accessible.
Removed ugly margins.
Signals cannot be declared virtual.
Let others know about changed progress.
Don't compare unsigned and signed ints.