storage: stop referencing variables from a lambda's parent scope

If a variable is referenced from outside a lambda expression, it is passed along with a struct pointer that could lead to reference cycles and double counting.

Even though, it does not happen here, there is no guarantee it won't in the future, and in fact it will with the GTK4 port.

In this case, the variable is also the sender of the signal. Therefore, the sender can be used instead.