Refactor event admin

[rlenferink]

The event admin currently does not work anymore. Refactor the event admin so that it works again and that:

• It created and export a event_admin_api interface library target containing the api
• Update api so that for event admin no event struct is needed -> i.e. signature celix_status_t (postEvent)(event_admin_t *event_admin, const char *topic, propeties_t *props);
• event admin api only contains a sendEvent and postEvent method.
• For sendEvent and postEvent the caller is responsbile for freeing the properties
• For postEvent the event admin will create a copy of the properties
• The eventHandle api contains one method with the signature: celix_status_t (*handleEvent)(void *handle, const char *topic, const properties_t *props);
• For event handler the topic and properties arguments are only ensure to be valid during the executing of the handleEvent method.

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Original creation date: 09/Apr/2018
Reporter: @pnoltes

[PengZheng]

Replaced by #515