Save magnet queue whenever the queue is updated
I.e. when magnets are added to the queue. Previously the queue was only saved when KTorrent was closed, so all magnets added in a session were lost if KTorrent crashed.
Test-plan: import a stale magnet link then send a SIGHUP to KTorrent and restart the app. Previously the magnet link did not appear on the magnets list after the restart, now it does.