gtksourceview: use tab-width for visual column
gtksourceview: use tab-width for visual column

We need to ensure we use the tab-width here instead of the indent-width
because we've already inserted tabs (as opposed to querying how much to
potentially indet with a \t).

Fixes #64

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