player: add window-id property

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

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