command: add new property 'title'

Add new property 'title' which indicates current playing title of disc.
This property is useful when using a stream whose title can be changed during playback, e.g., dvdnav. Since current mpv does not provide STREAM_CTRL_GET_CURRENT_TITLE for dvdnav, this commit
also contains implementation of STREAM_CTRL_GET_CURRENT_TITLE for dvdnav stream.

I made title property as a read-only property because the title can be specified by filename(e.g., dvdnav://1).