Do not quit Kleopatra when user chooses to just close the main window

In this case we do not want to accept the close event (as in the case
when the user closes the main window). Otherwise, Kleopatra quits
despite quitOnLastWindowClosed being false (for non-elevated users).

(cherry picked from commit 57359f6296ec2acde664b9f9b31e83dd4d4e1c3f)

• GnuPG-bug-id: T7050