Creates a list of paths of all selected items.
Additionally, if you are planning on modifying the model after calling this function, you may want to convert the returned list into a list of `GtkTreeRowReferences`. To do this, you can use TreeRowReference.
To free the return value, use:
g_list_free_full (list, (GDestroyNotify) gtk_tree_path_free);
A `GList` containing a `GtkTreePath` for each selected row.