Expose an API for adding ISearchProviders

CanvasView.m_Search isn't exposed in any way to external usage, so there's no way to call AddSearchProvider on it while loading a graph with custom providers.

Implement something nice to use - probably just a proxy method on CanvasView so someone can do something like:

using UnityEditor;
using UnityEngine;
using BlueGraph;
using BlueGraph.Editor;

namespace BlueGraphSamples
{
    [CustomEditor(typeof(ExperimentalGraph))]
    public class ExperimentalGraphInspector : GraphEditor
    {
        public override GraphEditorWindow CreateEditorWindow()
        {
            var window = base.CreateEditorWindow();
            window.Canvas.AddSearchProvider(new MyCustomProvider());
            return window;
        }
    }
}

McManning / BlueGraph

Expose an API for adding ISearchProviders #29