f/shells: added shouldSave property to shells.
f/shells: added shouldSave property to shells.

shouldSave adds the ability to prevent saves from happening even
when save chain of events are dispatching. This is useful, for example,
when swapping shells. Without this, swapped-out ``shell.EditView`` still
save through, which is incorrect when ``shell.EditView.onSave`` include
shell-specific logic.

athenalabs/acorn-player