main: Load extensions as late as possible

Since we want extensions to be able to be loaded at any time, that
means that the startup case should be as close to the live-time
install case.

https://bugzilla.gnome.org/show_bug.cgi?id=682822

js/ui/main.js

@@ -233,11 +233,6 @@ function start() {
         global.screen.override_workspace_layout(Meta.ScreenCorner.TOPLEFT,
                                                 false, -1, 1);
 
-    if (sessionMode.allowExtensions) {
-        ExtensionDownloader.init();
-        ExtensionSystem.loadExtensions();
-    }
-
     if (sessionMode.hasRunDialog) {
         Meta.keybindings_set_custom_handler('panel-run-dialog', function() {
            getRunDialog().open();
@@ -286,6 +281,11 @@ function start() {
     global.screen.connect('restacked', _windowsRestacked);
 
     _nWorkspacesChanged();
+
+    if (sessionMode.allowExtensions) {
+        ExtensionDownloader.init();
+        ExtensionSystem.loadExtensions();
+    }
 }
 
 let _workspaces = [];