Correctly list the open programs when trying to quit

Summary:
Uniquely iterate over the sessions instead of checking
for all sessions inside of a terminal window. The tabs in
split view will have duplicate sessions.

Test Plan:
Open Konsole, run any application that will not quit, like top
split screen, duplicating the Views, close konsole.

top will appear in the list of applications that are opened twice.

Reviewers: hindenburg, Konsole

Reviewed By: hindenburg, Konsole

Subscribers: konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D13364