ruqola: quit on closing the window

Summary:
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

Reviewers: mlaurent

Reviewed By: mlaurent

Differential Revision: https://phabricator.kde.org/D11386