csharp: Add helpers to get/set Values from Objects

csharp: Add helpers to get/set Values from Objects

Summary:
The user can construct an `Eina.Value` from a plain C# `object`, using
reflection to get the correct type of object and construct the correct
underlying C value.

Also added the `Unwrap()` method to return a C# object representing the
wrapped value.

Both operations are useful when using `Eina.Value` to
Get/Set values from `PropertyInfo` targets as in

```
var v = new Eina.Value(propInfo.GetValue(sourceObj));
...
propInfo.SetValue(targetObj, v.Unwrap());
```

Currently, containers are not supported. It will be added in a following
commit.

Depends on D9270

Reviewers: felipealmeida, vitor.sousa, segfaultxavi

Reviewed By: vitor.sousa

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9272