currently only supported on x11.
one practical use-case of this is wanting to embed something (such as
dmenu) into the mpv window to use as a menu/selection. there might be
other use-cases as well (e.g doing some shenanigans with `xdotool` or
whatnot).
it's currently possible to:
* listen for 'current-window-scale' change (to check if the
window has been created or not)
* call an external tool like `xdo` or `xdotool` and grab the xid
from mpv's pid.
however it adds unnecessary dependency on external tools when mpv is
fully capable of easily providing this information.
closes: #10918
NRK
25b66256d7