Quite a simple fix, we just assign the account-specific
`PositionTracker` to the level line's `._on_level_change()` handler
instead of whatever the current `OrderMode.current_pp` is set to.
Further this adds proper pane switching support such that when a user
modifies an order line from an account which is not the currently
selected one, the settings pane is changed to reflect the
account and thus corresponding position info for that account and
instrument B)