ObjectExplorer & Changes to PluginHost/Engine

- Added a functional ObjectExplorer Plugin PoC (`Yodii.ObjectExplorer.Wpf`), copied and trimmed from Yodii.Lab, with a WPF Demo (`Yodii.ObjectExplorer.WpfHostDemo`). Note: The new plugin *requires* a WPF context/app running.
- `PluginProxy`'s `TryLoad()` method will no longer throw an `ArrayOutOfBoundsException` when facing a Plugin constructor with parameters containing a non-Service type preceding a Service-type. See `Yodii.Host/Plugin/PluginProxy.cs`.
- When calling `YodiiEngine.Stop()`, the registered host will now disable *all* plugins. See `Yodii.Engine/YodiiEngine.cs`.