display-settings: Search for clonable modes only by resolution

`xfce_randr_clonable_mode()` must now return one mode per output,
because they are no longer the same modes (= resolution + refresh rate)
in general, only the resolutions are the same.

Fixes: #372
MR: !84
2 jobs for !84 with display-settings-clonable-modes in 2 minutes and 49 seconds (queued for 2 seconds)
latest merge request