Rework ui_dispatch tests to avoid need for Qt

[mdickinson]

Since #1788, we have only one test module that makes use of the Qt event loop. That test module contains tests for the behaviour of handlers that use dispatch='ui' mechanism to redispatch off-thread notifications to the ui thread.

This PR reworks that test module, with some significant collateral damage along the way.

In detail:

• reworks that test module (test_ui_notifiers) to avoid the need for the Qt event loop; instead, it tests against a ui_handler based on asyncio, which redispatches to the running asyncio event loop
• adds a get_ui_handler counterpart to set_ui_handler, and exposes both functions in traits.api
• adds type hints for get_ui_handler and set_ui_handler
• removes two public module globals from trait_notifiers: ui_handler has been made private, while ui_thread is removed altogether
• fixes a bug where ui dispatch didn't do the right thing (PR #1740 was incomplete; this bug should have been caught at review time on that PR)
• makes another couple of drive-by cleanups, removing a very old check for threading.local() being a dict (which it hasn't been in living memory), and tidying up some uses of thread identity.

[mdickinson]

@flongford Do you have bandwidth for review? This is part of a longer-term goal of insulating Traits from changes in Qt / PySide, and eventually decoupling Traits and TraitsUI entirely.

[mdickinson]

@flongford I've responded to comments above and made one minor update, for better exception handling.