Multi-selection and deselection using the shift key and an uncheck all box

[prototyperspective]

This would be very useful for #163 and #173 and because it's already possible to run batch statements for changing the files' wikidata items I wonder why one can't already uncheck items using the shift key.

By default, all items are checked. A way to quickly uncheck all (or check all again) would be useful even once there is a way to multiselect using the shift key.