add a get_indices_with_depth method to GtkTreePath with proper GI array annotations

add a get_indices_with_depth method to GtkTreePath with proper GI array annotations

* get_indices does not return a length so we can not annotate it to return an
  array in bindings that use GObject Introspection
* this method is the same as get_indices except it takes an int * as the depth
  parameter which we can then use in the array annotation
* in C this function returns an integer pointer array and updates depth to the
  number of integers in the array
* in a GI binding this returns the native array type for the bound language
  (e.g. in PyGI this returns a list of integers)