scene: Add layer_shell_v1 helper

This helper behaves similar to the xdg_shell helper, and additionally provides a little assistance for positioning and exclusive_zone management.

This is a light helper, and does not control layer ordering or reconfigure on change. It is up to the compositor to:

1. Provide layer ordering
2. To reconfigure on commit that changes state
3. To ensure that surfaces with an exclusive_zone are configured first
4. To apply the final usable_area to its other surfaces as appropriate

cc @emersion