Try to make monitor IDs stable between X11 and Wayland

@Tamaranch, just a heads-up here -- I want the monitor IDs to be the same between X11 and Wayland if possible (my new xfdesktop icon position tracking uses these IDs; would be nice if you switch between X11 and Wayland and your icons are in the same places automatically). Since we can't get the raw monitor EDID on Wayland like we can on X11, I think just hashing the make+model+serial should be fine. In places where there is no serial number (or no make and model), I tack on the connector name instead.

wlroots uses libdisplay-info to parse EDID and get the make, model, and serial numbers, and XfwMonitorX11 does the same, so the strings should end up being the same. Non-wlroots might do things differently, but not sure we can handle every case.

I was thinking about an extension to wl_output/xdg_output that our (future) compositor could have that would provide the EDID, but I doubt that would ever get standardized, and it would be nice if things would work well on most/all desktops, not just the compositor that we haven't even written yet...