outputs: Pass output updates

Introduce a callback in `outputs::Outputs` that calls on every `wl_output.done`, and a dummy consumer in `ui_manager`.

This is sufficient to detect display height changes.

src/ui_manager.h

@@ -7,7 +7,7 @@
 
 struct ui_manager;
 
-struct ui_manager *squeek_uiman_new();
+struct ui_manager *squeek_uiman_new(struct squeek_outputs *outputs);
 void squeek_uiman_set_output(struct ui_manager *uiman, struct squeek_output_handle output);
 uint32_t squeek_uiman_get_perceptual_height(struct ui_manager *uiman);