Set window flag on incoming message
From the [FreeDesktop specifications](https://specifications.freedesktop.org/wm-spec/wm-spec-latest.html#URGENCY): > Windows expecting immediate user action should indicate this using the urgency bit in the WM_HINTS.flags property, as defined in the ICCCM. Alternatively, we could use `_NET_WM_STATE_DEMANDS_ATTENTION`. Sub issue of #31.
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