Use xorg 'monitor' the right way.

Hello,

the existing code makes the assumption that there is a 1:1 relation between an "output" and a "monitor" in xorg-server / randr. This assumption is wrong by the randr specification but due to a bug in xorg-server randr implementation it has technically be true for a long time. With the merge of https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/981 it will be possible to create more than one monitor on an output.

This PR corrects the assumption by using the randr monitor name rather than the randr output name for gtk's monitor objects.

There's a large discussion in #2013 (closed) about this issue. The patch here along with the merged patch to xorg-server solves this issue ans allows to split up a very large display into multiple virtual monitors.