vo: add vo_drop_files()

Intended to be used for GUI drag & drop.
x11: implement dropping files on the VO window

Drag and drop is pretty complicated (just note how the number of Atoms
in use almost doubles), so I'm not sure whether this works everywhere.

This has been written by looking at the specification [1] or what looks
like the specification, and some external example code [2]. (The latter
one has no code license, but we didn't copy any code.)

We completely ignore the "requirement" of the spec. that the filename
"must" include username and hostname, e.g. "file://user@host/path/file".
In theory, this is required because X is network transparent, but at
this point the so called network transparency is a complete joke, and
Konqueror for one didn't include hostnames in "file://" URIs.

Tested with konqueror as drop source.

[1] http://www.newplanetsoftware.com/xdnd/
[2] http://www.edwardrosten.com/code/dist/x_clipboard-1.1/paste.cc
vo: dropping subtitle files on the VO window adds them as subtitle files

Note that we don't try to be clever about detecting the files as
subtitles: we just check the file extension. We could go all the way and
check the files by opening them with a demuxer, but that would probably
do more bad than good.