local-metadata: Add support for EPISODE_TITLE metadata

https://bugzilla.gnome.org/show_bug.cgi?id=741207
thetvdb: Add support for EPISODE_TITLE metadata

https://bugzilla.gnome.org/show_bug.cgi?id=741207
local-metadata: tests using EPISODE_TITLE metadata

https://bugzilla.gnome.org/show_bug.cgi?id=741207
thetvdb: tests using EPISODE_TITLE metadata

https://bugzilla.gnome.org/show_bug.cgi?id=741207
Updated Polish translation