compositor/main: put remoting/pipewire outputs to the right

Similar to what we have to all other backends/outputs, do the same
for the remoting/pipewire plug-ins and align them to the right.

This way we can move the plug-ins loading after
weston_compositor_flush_heads_changed(), much closer to the other module
loading which seems a more suitable place.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>