Clean up handling of libquvi (which resolves URLs of streaming sites
into URLs to the actual media playable by mpv). Move the code out of
open.c to quvi.c, and invoke it explicitly from mplayer.c, instead of
trying to resolve every filename passed to open_stream().
This allows easily passing metadata from the quvi context to the
frontend. Expose QUVIPROP_PAGETITLE as "media-title" property, and use
that instead of "filename" for the mplayer window title. (For YouTube,
this is the video title.) It's cleaner too.
Handle a potential reliability issue: check quvi_getprop return values.
Since open.c contains barely anything but the open_stream() stub, move
that to stream.c and delete open.c.
