- Add two new stage caches for unshared stages.
- These are necessary to be able to find unshared stages when toggling between shared and unshared.
- Do *not* pass an explicit sesssion layer when opening a stage unless unshared stage is explicitly desired.
- That is because the cache takes into consideration the session layer if given as an argument, which would unshare layers unintetionally.
- In particular, choosing to target the session layer (which is UI consideration) would indirectly make the stage unshared.
- Properly reload unshare stage and toggle between shared and unshared stage by transferring the content of the session layer.
- Clarify with comments the design and behaviour of the stage proxy shape.
- Adapted AL code to conform to the new API.
- Add unit test for session preservation

…usly set session layer.

- Fix typos.
- Simplify code.
- More efficient ref-counted pointer passing.