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.