ui: add CallbackManager for weak-referenced bound-method callbacks

[deanlee]

Adds a CallbackManager that stores bound-method callbacks as weak references and automatically skips those whose objects have been garbage-collected. This prevents callback lists from keeping UI objects alive and avoids memory leaks.

Note: Clients no longer need to manually remove callbacks—dead callbacks are ignored automatically.

A separate PR can add a SingleCallbackManager for places (e.g., dialogs, buttons) that only need single weak-referenced callback.

[github-actions]

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[sshane]

I introduced a new bug that wiped the wifi menu's back callback on hide in #36767 avoiding weakref. Weakref seems like the standard approach to callback/event listener setups in Python (our callbacks) @adeebshihadeh. Done well, you won't have to be careful about callbacks keeping a closed Widget in memory

https://blog.codingconfessions.com/i/172320360/weakset

[sshane]

Why do we have to make everything methods?

[deanlee]

Because WeakSet only supports weakly-referenced bound methods, adding lambdas or plain functions would complicate the design. Supporting bound-method callbacks is sufficient here, and we can use a separate class when we need more flexible callback types for a single-callback case.

[sshane]

Does this handle single callbacks added everywhere? For ex. the keyboard using 500K every time you open it from the back callback I assume

[deanlee]

Does this handle single callbacks added everywhere? For ex. the keyboard using 500K every time you open it from the back callback I assume

Here is a draft for single callbacks: #36788. Supports bound methods, functions, and lambdas.