POC: Rework toolkit support

[mdickinson]

This PR does a fairly large amount of refactoring and renaming, but its focus is on reworking the toolkit support.

The main goals are:

• Fix an architectural smell, namely that the toolkit magic was pervasive, going as far down as the message routing implementations: just importing multithreading_router made use of the toolkit machinery, for example.
• Be able to run the test suite against all installed GUI backends - wxPython, Qt, asyncio - in a single run. (It's still not advisable to try to run against multiple different Qt backends in a single run.)

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EDIT 2021-04-28: Most of the real content of this PR has now been extracted into separate PRs (all of which have been merged): #306, #307, #308, #309, #312, #314, #315

What's left is:

• timeout cleanup - already tracked in Clean up test timeouts #310
• changing the GuiTestAssistant to use the asyncio tests by default instead of the ETS toolkit. That is a change I want to make, but I'd be happier if we had toolkit-specific tests for the EventLoopHelper first, otherwise we risk some of that code going untested. (Done in Add tests for each of the IEventLoopHelper implementations #319.)

The EventLoopHelper may need to grow some new methods, for example a thread-safe method that sets a trait later, when the event loop is running. (Also done in #319.)

[mdickinson]

• I'd be happier if we had toolkit-specific tests for the EventLoopHelper first, otherwise we risk some of that code going untested.

Done in #319.

[mdickinson]

The final piece of this PR has been recreated as #321. Closing this one.