Maximilian Schiller authored Oct 17, 2018 and Eike Hein committed Oct 22, 2018

Summary:
MainWindow should inherit from KMainWindow and not from QMainWindow
because QMainWindow does not call the queryClose function upon closing.
This disabled the "really quit?" dialog which should come up if more than one session
is currently open.
Furthermore makes Yakuake properly quit when requested via the "Quit" action or the dialog by
making the QApplication quit if the last Window Closes.

BUG:  398425

Test Plan:
1. open yakuake
2. press ctrl + shift + q

without the patch yakuake just hides again (press F12 to confirm) with the patch it properly quits.

1. open yakuake
2. open multiple (>1) sessions (bottom left "+" symbol)
3. press ctrl + shift + q

without the patch yakuake just hides with the patch a dialog comes up and asks "really quit?"
on pressing "quit" yakuake properly quits.

Reviewers: hein, alexeymin, #kde_applications

Reviewed By: hein

Subscribers: tcanabrava

Differential Revision: https://phabricator.kde.org/D15535