ruqola: quit on closing the window
Is there any reason to make quitting so difficult (systray, quit,
confirm)? It's not like it's going to lose any data...
Either Ctrl+Q should work (but I couldn't find how to do that),
or simply closing the app could quit, unless this is against the KDE
Style Guide, for apps with a systray icon?
Test Plan: Alt+F4, ruqola quits
Reviewed By: mlaurent
Differential Revision: https://phabricator.kde.org/D11386