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.

(cherry picked from commit ca3440aa1e9e7748713552a2a0103ae7decdb1d1)
app: always use <Primary>D for the shortcut in the quit dialog

(do not take it from the button's mnemonic)

(cherry picked from commit 25c4acbd157e183dd8135b9d40fdd3140198cd95)
app: add the same <Primary>D shortcut in the image close dialog

and change the button label to "_Discard Changes" so it's the same as
in quit.

(cherry picked from commit 9057b63c1b5390289ef097166de28eaea26b9660)