TextView: Fix gesture coords with :border-width >0

TextView: Fix gesture coords with :border-width >0

Container:border-width caused the x/y coords converted to iters to be
offset inwards by that width, breaking positioning/selecting by gesture.

So, subtract :border-width in widget_to_text_window_coords(). This fixes
gesture positions, & plays fine with :margin & CSS margin/border/padding

N.B.: This is not to endorse :border-width. It’s gone in GTK+ 4 & weird
on a TextView: it’d be more intuitive to – if you must! – set it on the
TV parent. Really, please just use CSS instead. Still, it’s easy to fix.

https://bugzilla.gnome.org/show_bug.cgi?id=759725