request: add ondestroy handler about libui

[andlabs]

I could probably do this soon, sure. (Right now my Go bindings keep track of child controls and manually calls Destroy() on them as well. Perhaps with this commit I wouldn't need to do that...?)

[joaoventura]

This may also be a problem for a python wrapper..

[gnarz]

I have started along a similar venue, but that was getting way too complicated. An ondestroy handler seemed like a much simpler approach to me. As all control calls go through one function, this shouldn't be too hard.

[DemiMarie]

It should be per-widget. A global handler is not sufficiently flexible.

[pcwalton]

This is wanted for my Rust bindings as well.

What I'm doing now as a workaround is (a) maintaining a list of windows; (b) leaking all controls until uiUninit() is called, at which point I iterate over all the windows and destroy them; (c) having every method that calls into the FFI ensure that uiInit() has been called and uiUninit() has not been called. This essentially creates a weak reference semantics for controls, but it's one that leaks controls until the entire UI system has been shut down.

Another alternative to this approach would be to reference count controls. Effectively, this would make Control objects have strong reference semantics instead of weak reference semantics (as in this proposal). In GTK and Cocoa, all controls are already reference counted anyway, so this doesn't add much overhead.

[andlabs]

Merged with #325.