Proposed Session Restore API Change

Current restore methods:

• restore_snapshot_new() - allocates memory for objects without calling __init__()
• restore_snapshot_init() - initializes new object, equivalent to __init__() but using session data.

Proposed restore methods:

• restore_snapshot() - creates object and sets its state. Class method.
• set_state_from_snapshot() - sets state of an existing object saved in session.

I've tried this new API on current ChimeraX, replacing all session restore code. Encountered no problems.

I think we should try at least prototype scene saving using session API before we freeze our session APIs. I plan on trying the set_state_from_snapshop() method to restore scenes.