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).
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