Add support for positioning clients in outputs

Some clients offer the capability to position themselves at absolute X/Y locations in an output, or relative to output's bounds. From the user perspective, this is usually accomplished with a --geometry shell option or a similar configuration parameter (see example here). While X11 supports this behavior, to my knowledge similar implementation does not exist in xdg-shell or any other Wayland protocols.

I am attaching a reference to mpv issue, which lead to the creation of this proposal.