LibCore: Move event queueing to a per-thread event queue

Instead of juggling events between individual instances of
Core::EventLoop, move queueing and processing to a separate per-thread
queue (ThreadEventQueue).

Userland/Libraries/LibCore/CMakeLists.txt

@@ -30,6 +30,7 @@ set(SOURCES
     System.cpp
     SystemServerTakeover.cpp
     TCPServer.cpp
+    ThreadEventQueue.cpp
     Timer.cpp
     UDPServer.cpp
     Version.cpp