w32_common: prevent decoding certain keys twice

This is a bit of a hack, but in order to prevent TranslateMessage from
seeing WM_KEYDOWN messages that we already know how to decode, move the
decoding logic to the event loop. This should fix #476, since it stops
the generation of extraneous WM_CHAR messages that were triggering more
than one action on keydown.
msg: terminal OSD uses stderr, not stdout

This is more correct. E.g. if you do "mpv file.mkv > /dev/null", stdout
will not be a terminal, but stderr (used by terminal OSD and status
line) is.
msg: update comment