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/WebContent/CMakeLists.txt

@@ -6,6 +6,7 @@ set(WEBCONTENT_SOURCES
     ${WEBCONTENT_SOURCE_DIR}/PageHost.cpp
     ${WEBCONTENT_SOURCE_DIR}/WebContentConsoleClient.cpp
     ${WEBCONTENT_SOURCE_DIR}/WebDriverConnection.cpp
+    ../EventLoopImplementationQt.cpp
     ../EventLoopPluginQt.cpp
     ../FontPluginQt.cpp
     ../ImageCodecPluginLadybird.cpp