extensionPrefs: Fix opening extension preferences on startup

extensionPrefs: Fix opening extension preferences on startup

gnome-shell-extension-prefs supports opening a specific extension's
preferences directly from the command line by passing the UUID.
However this broke when extension loading was changed to be processed
asynchronously, as no extension has been loaded when the command
line argument is processed. Fix by deferring opening the extension's
preferences until all extensions have been loaded.

https://bugzilla.gnome.org/show_bug.cgi?id=694858