command: make 'disc-title' property writable

This commit makes 'disc-title' property writable using STREAM_CTRL_SET_CURRENT_TITLE.
This commit also contains implementation of STREAM_CTRL_SET_CURRENT_TITLE for stream_bluray.
Currently, 'disc-title' is writable for stream_dvdnav and stream_bluray and stream_dvd is not supported.

I couldn't find the way to implement STREAM_CTRL_SET_CURRENT_TITLE for stream_dvd.