app: make quitting by keyboard shortcuts easier

This mitigates the pain of having to confirm quitting when images have
only be exported but not saved: Introduce a <Primary>D shortcut in the
quit dialog and tell about it using a hint label at the bottom, so
quitting with dirty images is now a matter of pressing <Primary>Q
<Primary>D which is much easier to memorize than <Primary>Q
<Mnemonic>D.