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)
Gtk UI: Fix UnicodeDecodeError after downloads are finished (bug 1834)

When all downloads/syncs are finished, gPodder displays a summary of
downloaded episodes, having cut the titles if they are too long.
However, Russian descriptions are regular, non-unicode python strings,
and gPodder may cut only a part of a multi-byte UTF-8 sequence. It
causes an exception like this:
UnicodeDecodeError: 'utf8' codec can't decode byte 0xbe in position 51:
invalid start byte

This patch fixes that by converting the title to a unicode string if
it's not unicode.

This bug is similar to bug 1825, commit
e1ce9b0551d6380514d255ec03d3609c574860c8.
Merge pull request #102 from pluton8/bug1834_unicodeerror_finisheddownloads

Gtk UI: Fix UnicodeDecodeError after downloads are finished (bug 1834)