Fix missing notification title found by -Wswitch-enum

Summary:
When capturing a screenshot in background mode or when
Quit after Save or Copy is checked, Spectacle shows a notfication.

Normally that notification has the capture mode set as the title, but
for Window Under Cursor it was missing (in case
Capture the current pop-up only was unchecked too).

This was due to one missing case in a switch statement, found by using
stricter compiler warnings.

FIXED-IN: 18.08.1

Test Plan: spectacle -ub should include a title in the notification again.

Reviewers: Spectacle, ngraham

Reviewed By: Spectacle, ngraham

Differential Revision: https://phabricator.kde.org/D14716