display-settings: Unused possibilities of the wlr-output-management protocol on Wayland

Two things were left out of the Wayland port, because they don't have their X11 counterparts, but which could be useful in one way or another:

• Custom mode. One use comes to mind to address #18: instead of playing with scales as suggested there, simply set the largest output's custom mode to the smallest output's preferred mode. Not sure that adding the ability to set a custom mode in the UI is relevant though.
• Adaptive sync, aka Variable Refresh Rate or VRR. This could perhaps be added to the UI.