Need to improve documentation around glib wrapper types

[johncf]

Or more generally, "How gtk memory management works."

The most relevant write up as of now is: glib_wrapper! docs

The most important parts that are missing:

1. What are the semantics of Boxed, Shared and Object wrappers
   • Which are the traits implemented by each wrapper.
2. How is "inheritance" implemented using glib_wrapper!
   • Is it possible to derive a widget such as DrawingArea so as to add custom fields which can be accessed during event callbacks.
3. Motivation behind creating *GlibPtr* traits, and their behavior.
4. Details about thread-safety.

[GuillaumeGomez]

I greatly approve!