opensubtitles: Add ::finalize implementation
opensubtitles: Add ::finalize implementation

The source has some private data to free when an instance goes away.

jasuarez/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins