sessionMode: Add allowExtensions property

Add a sessionMode.allowExtensions property, which determines whether
installed and enabled extensions should be loaded or ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

js/ui/main.js

@@ -136,9 +136,6 @@ function _initUserSession() {
 
     global.screen.override_workspace_layout(Meta.ScreenCorner.TOPLEFT, false, -1, 1);
 
-    ExtensionSystem.init();
-    ExtensionSystem.loadExtensions();
-
     Meta.keybindings_set_custom_handler('panel-run-dialog', function() {
        getRunDialog().open();
     });
@@ -229,6 +226,11 @@ function start() {
     if (sessionMode.sessionType == Shell.SessionType.USER)
         _initUserSession();
 
+    if (sessionMode.allowExtensions) {
+        ExtensionSystem.init();
+        ExtensionSystem.loadExtensions();
+    }
+
     if (sessionMode.hasOverview) {
         Meta.keybindings_set_custom_handler('panel-main-menu', function () {
             overview.toggle();

js/ui/sessionMode.js

@@ -12,12 +12,14 @@ const _modes = {
              hasAppMenu: false,
              showCalendarEvents: false,
              allowSettings: false,
+             allowExtensions: false,
              sessionType: Shell.SessionType.GDM },
 
     'user': { hasOverview: true,
               hasAppMenu: true,
               showCalendarEvents: true,
               allowSettings: true,
+              allowExtensions: true,
               sessionType: Shell.SessionType.USER }
 };