Pyface should not import ui_handler and set_ui_handler from `traits.trait_notifiers`

[mdickinson]

Pyface currently imports ui_handler and set_ui_handler directly from traits.trait_notifiers, in the qt and wx toolkit implementations (in the init.py modules).

Instead of this, it should import get_ui_handler (available since Traits 7.0.0) and set_ui_handler from traits.api, and use those.

For backwards compatibility, we should have a fallback path to reading ui_handler directly if get_ui_handler is not available.