Master issue for global event handling suggestions and improvements

[andlabs]

An assortment of suggestions to event handling. Codependent on #321 and #325.

• Instead of having a separate function for every event, have a single function and the event passed in as a constant (maybe)
• The ability to programmatically call an event handler for both events and menu actions; or alternatively, a way to specify that they should be called when programmatically changing something (maybe)
• Likewise, but for uiQuit() (maybe)
• Drop uiAreaHandler and switch to event callbacks for it, passing the data argument normally instead of extending the structure; or alternatively, having the data argument be part of uiAreaHandler itself (maybe for the latter)

This topic will be unlocked when I merge existing issues in.

[parro-it]

I recently discovered the way libui-node implements event handlers leaks memory.
That's because I never properly cleanup events callbacks when control are destroyed.

Trying to improve this stuff, I get stucked because I don't find a proper way to discover when a control is destroyed.

I would need one of these implemented in libui:

1. an onDestroy event fired by every control when they are destroyed.
   or
2. a new void* data element on uiControl struct (this would allow me to implement previous event on C++ side)

Without one this, I could solve the problem implementing a map data structure that associate every control with its C++ wrapper class, but I feel this solution would be both more difficult to implement, and more error prone.