Fix memory leak

Each Notification::notifications() call is allocating new objects which must be deleted by the caller:

==32157== 240 (44 direct, 196 indirect) bytes in 1 blocks are definitely lost in loss record 2,092 of 2,217
==32157==    at 0x4838424: operator new(unsigned int) (vg_replace_malloc.c:292)
==32157==    by 0x9858937: Notification::notifications() (notification.cpp:470)
==32157==    by 0x9814B3B: MailStoreObserver::publishedNotification() (mailstoreobserver.cpp:308)
==32157==    by 0x9818523: MailStoreObserver::actionsCompleted() (mailstoreobserver.cpp:333)
==32157==    by 0x981EFFF: MailStoreObserver::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_mailstoreobserver.cpp:97)
==32157==    by 0x4DCC43F: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3580)
==32157==    by 0x981BE6B: ActionObserver::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (moc_actionobserver.cpp:299)
==32157==    by 0x4DCC43F: QMetaObject::activate(QObject*, int, int, void**) (qobject.cpp:3580)
==32157==    by 0x4DD8C3F: QSingleShotTimer::timerEvent(QTimerEvent*) (qtimer.cpp:292)