Gtk UI: Fix device sync progress not updating (bug 1832)

When the verbose mode is on, and the database is big, it can take
several seconds for gPodder to log all excluded from sync episodes. At
that time, the update downloads list timer stops, because there are no
sync tasks yet. This patch ensures the download list is updated after
all the sync tasks are added to the download queue.
Merge pull request #101 from pluton8/bug1832_device_sync_progress

Gtk UI: Fix device sync progress not updating (bug 1832)