Improve stage edit target updates with refreshSystemLock callbacks

[jufrantz]

On refreshSystemLock, the stage's editTarget could be improperly set to the session layer when our callback unlocks a layers which is non-writable on disk. In such cases, in-memory layer editions should still be allowed without enforcing the session layer. We are encountering this issue in production and we need the editTarget selection to be preserved in such case.

Additionally, the editTarget update did not account for new locks that could be introduced by onRefreshSystemLock callbacks.

Included changes:

• Changed the timing of updateEditTarget calls within the refreshSystemLock command: jufrantz@33e48e2
• Added unit tests to validate refreshSystemLock behavior across various callback scenarios: jufrantz@c07344d