csharp: Fix free_cb calling under dotnet.

csharp: Fix free_cb calling under dotnet.

Summary:
dotnet's GC seems to be more agressive, showing some issues that usually
do not appear when running under Mono's.

This commit uses for free_cb's the same scheme we already use for
regular Eo refs, using a GC handle to keep the cleaning callback alive.

Reviewers: vitor.sousa, felipealmeida, woohyun

Reviewed By: vitor.sousa

Subscribers: segfaultxavi, cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8593