Properly handle discarding the dialog asking whether to override a
keybinding as canceling it rather than as allowing multiple identical
keybindings.

In the way, simplify and fix dialogs_show_prompt() not to perform odd
and useless response mapping that effectively go round back, and that
don't handle what the comment above it suggests.  Simply document it
can return GTK_RESPONSE_DELETE_EVENT and handle it in the caller side,
as it's a possibly valuable information.  Only one current caller is
affected, and it doesn't change anything as it doesn't change behavior
but only documents it.

Closes #714.