Fix the in-app preferences launcher (no longer working in 3.12+)

Gnome Shell's 5dedc5d8ba4f0e6fac8e4c62e558a0b30eb6f0f1 removed the functionality previously used to launch the Preferences: instead of mimicking what has been done in Gnome
Shell's 2d24536caf8cbd4d3c5aea705cddb57f1a23d7c9 and switch to GAppInfo's launch_uris(), go straight to the provided LaunchExtensionPrefs() method.
Note: still compatible with ~3.10.