display: support generic wlr-output-management-unstable-v1 (#1840)

The display config UI only applied changes for compositors with a
config-file backend (niri, hyprland, dwl).  For any other compositor
that supports wlr-output-management-unstable-v1 the "Apply Changes"
button was silently a no-op.

Add WlrOutputService.applyOutputsConfig() as a high-level apply that
mirrors the generateOutputsConfig() pattern of the existing services
but applies directly via the protocol instead of writing a config file.
Route the default case in backendWriteOutputsConfig() to it.

This enables using dms-shell as a wayland compositor for emacs wayland
manager (ewm).

quickshell/Modules/Settings/DisplayConfig/DisplayConfigState.qml

@@ -909,6 +909,9 @@ Singleton {
         case "dwl":
             DwlService.generateOutputsConfig(outputsData);
             break;
+        default:
+            WlrOutputService.applyOutputsConfig(outputsData, outputs);
+            break;
         }
     }