[settings] Rename DVDs category to Discs and add an option to specify the handling of blurays.
[FileItem] Also check for MovieObject.bdmv in IsBDFile().
[bluray] Use "disc.playback" setting to start bluray playback.
[*RFC*] Treat bluray directories like images and discs when playing them via the context menu.
Rename isDVDImage() to isDiscImage().