gutter: use signal group to track adjustments
gutter: use signal group to track adjustments

Rather than expecting to have the adjustments available when the widget
is created, we should delay setup and track changes as they occur.

Thankfully, we have GtkSourceSignalGroup now which makes that rather
trivial to be more complete in our support here.

Fixes #175

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview