view: use GtkSourceIndenter interface
view: use GtkSourceIndenter interface

This uses the indenter interface instead of the compute_indentation()
implementation from previous. Doing so allows for applications to provide
custom indentation providers that may differ from what GtkSourceView
provides internally.

Fixes #166

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