[Core] Enable file_completed_alert handling.

Without adding file_progress to the alert mask, the
TorrentFileCompletedEvent would never fire.

Closes: https://dev.deluge-torrent.org/ticket/3421
Closes: https://github.com/deluge-torrent/deluge/pull/370
Ref: https://libtorrent.org/upgrade_to_1.2-ref.html

deluge/core/alertmanager.py

@@ -46,6 +46,7 @@ class AlertManager(component.Component):
             | lt.alert.category_t.status_notification
             | lt.alert.category_t.ip_block_notification
             | lt.alert.category_t.performance_warning
+            | lt.alert.category_t.file_progress_notification
         )
 
         self.session.apply_settings({'alert_mask': alert_mask})