* app/core/gimpmarshal.list: add marshallers needed for the binding
  singnals.

* app/tools/gimptexttool.[ch]: add binding signals "move-cursor",
  "delete-from-cursor" and "backspace" and hijack GtkTextView's
  binding set to invoke them. Move code from the key_press() handler
  to the signals' default handlers. This is how it should work. In
  fact that code is #ifdef'ed away and we need an evil proxy
  GtkTextView to invoke the bindings on because of reasons stated in
  comments in the code. Ugly but works just fine.