GtkTextView: use pointer or Ref instead of _GtkTextIter struct (breaking)

[jwahlstrand]

The struct _GtkTextIter is not opaque, but users of this package are never going to want to modify its fields, which are literally all named "dummy". It makes way more sense to pass pointers and references to it than the struct itself, and it's more consistent with everything else in this package. Both GtkTextView and GtkTreeView sort of fell through the cracks when I was sorting out major issues back around version 0.3.

One could do the same thing with GtkTreeView and _GtkTreeIter, but that API is deprecated in recent versions of GTK4 and its functionality can be replaced with GtkListView, etc. So I am strongly inclined to leave it alone.

I plan to merge this and #49 pretty soon and release 0.7.0.