lines: avoid use of style-updated
lines: avoid use of style-updated

This API was removed in GTK 4, so we need to drop our use of it. Instead
we will watch for CSS changes and apply that.

Long term, this will work better as CSS is inhereted from parent widgets
before 4.0 is released.

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