Fixes: #U4-1546 - Moves IApplicationEventHandler to Umbraco.Core and creates UmbracoApplicationBase object in the Core. Changes

signatures of IApplicationEventHandler methods to accept an UmbracoApplicationBase object instead of UmbracoApplication. This allows
us to execute all IApplicationEventHandler's outside of the web context. Added unit test to support booting the application outside of the
web context.

src/Umbraco.Core/PluginManager.cs

@@ -396,6 +396,15 @@ namespace Umbraco.Core
         private readonly HashSet<TypeList> _types = new HashSet<TypeList>();
         private IEnumerable<Assembly> _assemblies;
 
+        /// <summary>
+        /// Returns all available IApplicationStartupHandler objects
+        /// </summary>
+        /// <returns></returns>
+        internal IEnumerable<Type> ResolveApplicationStartupHandlers()
+        {
+            return ResolveTypes<IApplicationStartupHandler>();
+        }
+
         /// <summary>
         /// Returns all classes attributed with XsltExtensionAttribute attribute
         /// </summary>