Add empty flag and use it to ignore empty sheets in report creation.
Do not show skiped sheets in progress bar when iterating.
Add "verified" and "reconized" attributes to the sheet.
Mark sheets as recognized after the recognition is done.
Only run recognition over new pages.

This also adds the "--rerun" option, which ignores any flags and
reruns the recognition.
Set newly recognized sheets to not be "verified".

This is important in the case where a --rerun happens.
Fire change notifications for sheet object modifications.
Add global flags to the widget GUI representation.
Remove global valid toggle, it is in the widget representation.
Set verified if Enter is used to go to the next image.
Fix sheet __setattr__ to always work for private properties.

Private properties should just be settable. They are not pickled and
and no notification is neccessary.
Merge branch 'stateflags'