Add plural form of translation string.

[neodyne]

Noticed a translation string without a plural form while working on bringing the Slovak translation of gPodder up to date.
I guess Renamed 1 downloaded episodes is passable in English, but it stands out in other languages, especially those with multiple plural forms (like Slovak):

Renamed 1 episode    -> Premenovaná 1 epizóda
Renamed 2-4 episodes -> Premenované 2 epizódy
Renamed 5+ episodes  -> Premenovaných 5 epizód

make messages will need to be rerun as well, to update messages.pot and individual translations.

[elelay]

merged, thanks!

[elelay]

By the way, do you have a strong opinion on #1551? If so I would like to ear from you :-)

[neodyne]

By the way, do you have a strong opinion on #1551? If so I would like to [h]ear from you :-)

@elelay Not necessarily a strong opinion, but I agree with "--:--", it gets the point across and doesn't have translation issues. Filesize / transfer speed should be translated (so Ko/s in French?, КБ/с in Russian etc.).

By the way, the Slovak translation in #1555 should be ready to be merged; I also submitted #1556, if you'd take a look.