More useful exit warning message

[sidequestboy]

Currently, when a background is previewed and not applied, clicking the close button brings up this message:

"You previewed an image without applying it, exit anyway?" N/y
where the close window button does the same thing as the "Yes" button

I believe a more intuitive (classic), and useful dialogue would be:
"You previewed an image without applying it, apply?" Y/n
where the close window button cancels the exit, without applying;
the Yes button continues the exit, after applying;
the No button continues the exit, without applying

i.e. the behaviour is identical to the classic "You have unsaved changes, save?" Y/n dialogue.

[daf]

Fixed in #33