Renamed `hook_storage_prop_shape_alter` to `hook_canvas_storable_prop_shape_alter`

At our first stable release of Drupal Canvas, we provided a non-documented hook_storage_prop_shape_alter hook for altering the candidate prop shape for. matching prop shapes.

This has been renamed to hook_canvas_storable_prop_shape_alter, with the same signature, and we documented it in canvas.api.php.

Code implementing the deprecated hook will get a deprecation warning, and will stop working once Drupal Canvas 2.0.0 is released.