LibCore: Big first step towards pluggable Core::EventLoop

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.

Userland/Libraries/LibConfig/Client.cpp

@@ -14,7 +14,6 @@ static RefPtr<Client> s_the = nullptr;
 Client& Client::the()
 {
     if (!s_the || !s_the->is_open()) {
-        VERIFY(Core::EventLoop::has_been_instantiated());
         s_the = Client::try_create().release_value_but_fixme_should_propagate_errors();
     }
     return *s_the;