Ladybird: Run the Core::EventLoop with a Qt backend

This patch adds EventLoopImplementationQt which is a full replacement
for the Core::EventLoopImplementationUnix that uses Qt's event loop
as a backend instead.

This means that Core::Timer, Core::Notifier, and Core::Event delivery
are all driven by Qt primitives in the Ladybird UI and WC processes.

Ladybird/CMakeLists.txt

@@ -83,6 +83,7 @@ set(SOURCES
     ${BROWSER_SOURCE_DIR}/History.cpp
     BrowserWindow.cpp
     ConsoleWidget.cpp
+    EventLoopImplementationQt.cpp
     HelperProcess.cpp
     InspectorWidget.cpp
     LocationEdit.cpp