Ladybird: Move the AudioThread class into its own file

This implementation detail of audio support in ladybird is a QObject
that needs moc'd by the moc tools. Putting it in its own file follows
the pattern we have for all the other QObjects in Ladybird.

Ladybird/WebContent/CMakeLists.txt

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