log: add basic utility to log to editor from native plugin

Need to clarify the various logging things:

• libvlcsharp logs (to Unity Editor console or stdout),
• libvlc logs (_libVLC.Log += (s, e) => UnityEngine.Debug.Log(e.FormattedLog);)
• now in-editor vlc plugin logs.

components that may want to publish logs:

• unity users script,
• libvlcsharp,
• vlc unity plugin,
• libvlc.

possible outputs:

• Unity Editor Log Console
• stdout,
• log file.

solution:

• register a log callback from UnityEngine.Debug.Log from the vlc-unity c# initialization script to the C++ vlc unity plugin.
• expose log functions from the vlc-unity plugin (one for logging to the Unity Editor, another to log to stdout).
• have libvlcsharp call into either of the exposed log functions (notably when an exception is thrown from a reverse callback).

I guess the user could want:

• to subscribe all logs in a single call,
• to selectively subscribe to some logs,
• to send the libvlc logs to the editor and to send the vlc plugin logs to a file?
• unity editor log color support, enable/disable