Use the new GtkTextView draw_layout vfunc to fix icon-cache rendering

Rather than draing things before and after chaining up to the
textview draw vfunc we use the new draw_layout vfunc to do
these draws at the correct place. This fixes drawing as the new
TextView now 1) draws the background, 2) uses an pixel cache, which
makes the previous approach not work.

https://bugzilla.gnome.org/show_bug.cgi?id=733871