Oak only uses KDDockWidgets' QtWidgets frontend. Building the QtQuick
frontend caused duplicate QML module registration when QtQuick.framework
initialized, resulting in a SIGABRT in QQmlMetaType::qmlInsertModuleRegistration.
- Set KDDockWidgets_FRONTENDS to qtwidgets in ext/CMakeLists.txt.
- Remove the -qmldir flags from macOS CD deploy since QML plugins are no
longer needed.