Ladybird: Allow posting events to the Qt event loop from other threads

Previously, a QTimer was used to start processing of our event queue in
the main Qt event loop. Unfortunately, QTimers are not thread-safe, and
disallow starting of a timer from a different thread than it was
created in.

Instead, use a dummy QObject to post a custom QEvent to the main loop
from whatever thread we like, and process our event queue when it is
received by our dummy object.

Ladybird/WebContent/CMakeLists.txt

@@ -9,6 +9,7 @@ set(WEBCONTENT_SOURCES
     ../AudioCodecPluginQt.cpp
     ../AudioThread.cpp
     ../EventLoopImplementationQt.cpp
+    ../EventLoopImplementationQtEventTarget.cpp
     ../FontPlugin.cpp
     ../HelperProcess.cpp
     ../ImageCodecPlugin.cpp