Add button to access preferences display

[bulletmark]

This extension displays plenty of info and has buttons for everything but the glaring omission seems to be that it lacks a button to get to it's own gnome-shell-extension-prefs configuration page. Please consider adding.

[eonpatapon]

The button is displayed when the connection fails. Otherwise the button is hidden as it's not something you would use regulary.

I don't want to clutter the interface more with this.