sessionMode: add a property to disable window manager menus

We will use this to disable menus in the initial-setup session.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/569

js/ui/sessionMode.js

@@ -25,6 +25,7 @@ const _modes = {
         hasWorkspaces: false,
         hasWindows: false,
         hasNotifications: false,
+        hasWmMenus: false,
         isLocked: false,
         isGreeter: false,
         isPrimary: false,
@@ -86,6 +87,7 @@ const _modes = {
         hasRunDialog: true,
         hasWorkspaces: true,
         hasWindows: true,
+        hasWmMenus: true,
         hasNotifications: true,
         isLocked: false,
         isPrimary: true,