dbusServices/extensions: Only show dialog when loaded

After the port to ESM, an extension's prefs.js file is imported asynchronously. An unintended side effect of that is that we now show the dialog before anything can be added to the window (either by the extension, or the fallback error UI).

The delay almost always won't be noticeable to users, but it's bad practice and prevents extensions from using some API that only works before the window is realized.

To address the issue, add a loaded signal to the dialog that allows the caller to postpone showing the window until the UI is ready.

Close: #7201 (closed)