Consider safety of almost all API calls

[kenz-gelsoft]

wx manages (almost all) wxWindows' pointers which is in widgets tree.

This means that any operation potentially makes any pointers invalid.

This should be expressed unsafe in Rust, how can we do this?

[kenz-gelsoft]

Marking following methods unsafe may make it programmer's responsibility

• destroy()
• any close operations of wxWindows (which results destroying wx managed pointers)

I think this is a reasonable compromise than other approaches like:

• Programmer checks all wx calls is safe by:
  • Making all calls returns Result and fallible
  • Checking every reference to wx-managed pointer by get()ing from wx::WeakRef.

Those approaches will increase run-time overhead unreasonablly. As a Rust library, this isn't desirable.

Providing unsafe and performant variants to whole APIs doubles the work for binding.