core: DownloadManager: abort running downloads upon shutdown

... and remove all incomplete local files of uncompleted downloads.

Connecting DownloadloadManager::shutdown() to QQmlEngine::quit()
should be perfectly save also for our singleton pattern approach: If
ever the QmlEngine deleted the DownloadManager intance prior to firing
the quit() signal (which does not seem to happen) we call into
shutdown() also from ~DownloadManager().
core: main: remove duplicate setIcon() call