Updated the PluginTypeResolver to have a generic way of instantiating the plugin types it has

already found. Have added some notes to some classes that need to be fixed up. Removed the PluginTypeResolver
from the ApplicationContext since it is a utility and isn't exposing application objects.

src/Umbraco.Tests/PluginTypeResolverExtensions.cs

@@ -7,7 +7,7 @@ namespace Umbraco.Tests
 	/// <summary>
 	/// Used for PluginTypeResolverTests
 	/// </summary>
-	public static class PluginTypeResolverExtensions
+	internal static class PluginTypeResolverExtensions
 	{
 		public static IEnumerable<Type> ResolveFindMeTypes(this PluginTypeResolver resolver)
 		{