WebAssembly: Event dispatcher mk II

The current event dispatcher implementation has some defects we'd like to correct:

• Make a QtCore-only event dispatcher, so that e.g. network can work with QCoreApplication.
• Don't inherit QUnixEventDispatcherQPA. Some of the virtuals we'd like to override are final on that class. Either re-implement or drop the features it provides (such as n:1 timer multiplexing).
• Remove the need for calling QWasmEventDispatcher::maintainTimers() at the end of all wakeups. Call it automatically on postEvent, registerTimer, and requestUpdate instead.
• Make sure the multithreading missing event loop wakeup bug gets fixed (QTBUG-75793)
• Support not calling app.exec(). Creating a Q[Gui|]Application instance should be sufficient.

Bonus

• Platform autotests for the event dispatcher
• Asyncify support