extensionSystem: Require Extensions app for updates

While we don't technically need the app to download and apply
updates, we do require it for notifying the user about available
updates and listing extensions with pending updates.

So instead of intransparently applying updates in the background
without the user noticing, disable updates altogether if the
Extensions app is not installed.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2346

js/ui/extensionDownloader.js

@@ -108,6 +108,9 @@ function gotExtensionZipFile(session, message, uuid, dir, callback, errback) {
 }
 
 function downloadExtensionUpdate(uuid) {
+    if (!Main.extensionManager.updatesSupported)
+        return;
+
     let dir = Gio.File.new_for_path(
         GLib.build_filenamev([global.userdatadir, 'extension-updates', uuid]));
 
@@ -126,6 +129,9 @@ function downloadExtensionUpdate(uuid) {
 }
 
 function checkForUpdates() {
+    if (!Main.extensionManager.updatesSupported)
+        return;
+
     let metadatas = {};
     Main.extensionManager.getUuids().forEach(uuid => {
         let extension = Main.extensionManager.lookup(uuid);

js/ui/extensionSystem.js

@@ -60,6 +60,11 @@ var ExtensionManager = class {
         ExtensionDownloader.checkForUpdates();
     }
 
+    get updatesSupported() {
+        const appSys = Shell.AppSystem.get_default();
+        return appSys.lookup_app('org.gnome.Extensions.desktop') !== null;
+    }
+
     lookup(uuid) {
         return this._extensions.get(uuid);
     }
@@ -481,6 +486,9 @@ var ExtensionManager = class {
     }
 
     _installExtensionUpdates() {
+        if (!this.updatesSupported)
+            return;
+
         FileUtils.collectFromDatadirs('extension-updates', true, (dir, info) => {
             let fileType = info.get_file_type();
             if (fileType !== Gio.FileType.DIRECTORY)