Preserve surface order when moving surfaces between layers

Use a single list of layer-surfaces and only adjust the layer instead of moving them between lists. This gives a deterministic layer ordering even when surfaces change between layers.

Commit messages have a bit more details.

I'm marking this as draft as I want to check the code paths for committed vs pending state and some other bits but wanted to get the idea out.

This helps phosh#764 (closed) as phosh doesn't need to worry about the relationship to other layers more than it does atm.

This is on top of !369 (merged).