Restructure CEF stuff in preparation for launcher packaging.

Robust.Client.CEF Renamed to Robust.Client.WebView since CEF should really be an implementation detail.
Content is no longer responsible for initializing and managing the module, this is done automatically by the engine.
WebView is initialized by declaring it in a manifest.yml file in the game resources. In the future the launcher will read this same file to manage WebView module versions.
CefManager has been made private and the content-visible API is now IWebViewManager.

Robust.Client.WebView/IWebViewWindow.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace Robust.Client.WebView
+{
+    public interface IWebViewWindow : IWebViewControl, IDisposable
+    {
+        bool Closed { get; }
+    }
+}