csharp: Fix event marshalling for value types
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.
Reviewers: woohyun, felipealmeida, vitor.sousa, segfaultxavi
Reviewed By: segfaultxavi
Maniphest Tasks: T7957
Differential Revision: https://phab.enlightenment.org/D8889