This was in sub/, because the code used to be specific to subtitles. It
was extended to automatically load external audio files too, and moving
the file and renaming it was long overdue.
This puts in place the machinery to merely append dropped file to the playlist
instead of replacing the existing playlist. In this commit, all front-ends
set this to false preserving the existing behaviour.
I don't like this function at all, but it's basically a trick to get the
input's mp_log instance in a case the mp_input_parse_cmd_strv() is
almost certainly not going to output anything. But still make it
somewhat more consistent with mp_input_parse_cmd_strv() - why force the
caller to always use MP_ON_OSD_AUTO?
Both X11 and Wayland support the same format for drag & drop operations
(text/uri-list), and the code for that was copied from x11_common.c to
wayland_common.c. Factor it out.
