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:

Reviewers: Spectacle, ngraham, davidre

Reviewed By: Spectacle, ngraham, davidre

Subscribers: broulik, ngraham, davidre

Tags: Spectacle

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