Add missing <numeric> include from ThumbNailer example code.
Add MainWindow.

Also remove remnants of UI from HelloInternet.
Move Benchmark into it's own files.
Create basic UI for demo.
Implement progress signal in Model.
Clear data after steps are complete.

This keeps memory usage down. Also, throw the correct exceptions.
Demo now converts the files. No view yet, though.
Progress now always counts to 100%, even on conversion errors.
Remember previous open file location.
Remove option to convert directories for now. Save output location.
Connect processing and the view.

Need to check for race conditions, though.
Implement sorting of the display using a QSortFilterProxyModel.
Simplify error reporting in Image.
Illustrate decorators by implementing PriorityDecorator.