extensionSystem: Add support for mode-specific extensions

As currently envisioned, the fallback replacement in 3.8 should be
a separate session at the login screen. As we will use extensions
to implement this mode, we need a way to specify extensions per
session rather than per user, so add a session-mode property for
extensions that should be loaded in addition to the user-defined
ones.

https://bugzilla.gnome.org/show_bug.cgi?id=689305

js/ui/sessionMode.js

@@ -18,6 +18,7 @@ const _modes = {
         showCalendarEvents: false,
         allowSettings: false,
         allowExtensions: false,
+        enabledExtensions: [],
         hasRunDialog: false,
         hasWorkspaces: false,
         hasWindows: false,