Count down the seconds until a screenshot is taken in the window title

Summary:
This way the user knows by the progressbar __and__ by the changing
number when the screenshot is going to be taken.

Test Plan: {F6876616}

Reviewers: #spectacle, ngraham, davidre

Reviewed By: #spectacle, ngraham, davidre

Subscribers: broulik, ngraham, davidre

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D21651