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/LibCore/CMakeLists.txt

@@ -11,6 +11,8 @@ set(SOURCES
     ElapsedTimer.cpp
     Event.cpp
     EventLoop.cpp
+    EventLoopImplementation.cpp
+    EventLoopImplementationUnix.cpp
     File.cpp
     IODevice.cpp
     LockFile.cpp