csharp: Fix event marshalling for value types

Summary:
It was wrongly assuming value types were passed by value.

As stated in the documentation, all arguments are passed with a single
level of indirection.

Fixes T7957

Reviewers: woohyun, felipealmeida, vitor.sousa, segfaultxavi

Reviewed By: segfaultxavi

Subscribers: cedric, reviewers, committers

Tags: efl

Maniphest Tasks: T7957

Differential Revision: https://phab.enlightenment.org/D8889