Column: added _ready _set_child_column methods

The _ready method is used to mark a column as ready, that is calling its
handler closure. That's why we need to store the closure for later use.
Handler: every handler now requires a ptr to its parent column
Tab: removed generate_parent_paths function

Creating of columns is now reverted back to its original form - creating
them in reverse order (that's why we no longer need the
generate_parent_paths function, nor the parent_paths static global
variable). This causes no problem due to Column's _ready method.
Type_factory: changed the signature of Type_product
List_dir: we're now setting the cursors for inactive columns too