extensionPrefs: Skip main window when launched with a UUID

extensionPrefs: Skip main window when launched with a UUID

The extension-prefs tool is used by gnome-tweak-tool and the
extensions web site to display preferences. However as those
already implement their own extension lists, the main window
is not useful in that context (to not say it is rather silly).

Just skip the main window and only show the specified extension's
preference dialog in those cases.

https://bugzilla.gnome.org/show_bug.cgi?id=730829