Using QEventLoop works for everything but it breaks *one* little feature
that we care about: automatically quitting the app when all windows have
been closed.
That only works if you drive the outermost main event loop with a
QCoreApplication instead of a QEventLoop. This is unfortunate, as it
complicates our API a little bit, but I'm sure we can think of a way to
make this nicer someday.
In order for QCoreApplication::exec() to process our own
ThreadEventQueue, we now have a zero-timer that we kick whenever new
events are posted to the thread queue.
The EventLoop is now a wrapper around an EventLoopImplementation.
Our old EventLoop code has moved into EventLoopImplementationUnix and
continues to work as before.
The main difference is that all the separate thread_local variables have
been collected into a file-local ThreadData data structure.
The goal here is to allow running Core::EventLoop with a totally
different backend, such as Qt for Ladybird.
