Added final callback after all listeners have been invoked.

[skywa04885]

When working with dynamic memory or object pools, and the number of listeners is dynamic, I needed to make possible that memory is freed or objects released safely after all listeners have been executed. I added this quickly (not sure if it's up to the standard or design you want), but might be a idea to implement this officially.

[andsmedeiros]

Thank you very much for your contribution! I feel your changes are very plausible and fill in an important usability gap in the framework.

That said, please make these few improvements for me to accept your PR:

• Un-commit .vscode directory
• Instead of invoking final_callback in run_signal_event(), invoke it in uel_evloop_run(). This way, we can also implement finalisers for other events later
• Change uel_event_config_signal signature to take the finaliser as parameter
• Document your changes in README.md
• Write unit tests for uel_signal_emit_with_final_callback()
• Re-generate the Doxygen docs and commit them (right now, your commit deletes our docs)

If you have any trouble, I will gladly help.