csharp: encapsulate some internal code of EoWrapper

csharp: encapsulate some internal code of EoWrapper

Summary:
Encapsulate some parts of EoWrapper making them less accessible to lib users.
This can avoid unnecessary and risky usage of code that is only intended for
internal usage.

`inherited` field was made private and renamed to `generated`. Now its value
can only be obtained through the `IsGeneratedBindingClass` property.

`handle` field was made private.

`eventLock` was renamed to `eflBindingEventLock`

`ConstructingHandle` property set was made private.

Constructors that are used to create new EFL# managed objects by wrapping a
preexisting eo handle now receive a specific struct wrapping the handle pointer.
This can avoid faulty interactions with the Reflection engine used only for
generated classes that implement this constructor.

Test Plan: meson test

Reviewers: lauromoura, felipealmeida, YOhoho

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9212