AppMan: Convert ApplicationInterface and StartupTimer from context properties to proper QML singletons

Currently, tools like the Qt Creator code model, qmllint, and qmlsc cannot correctly analyze ApplicationInterface and StartupTimer because they are injected using QQmlContext::setContextProperty().

Suggestions

• (Short-term) Converting this to qmlRegisterSingletonType() or qmlRegisterSingletonInstance() will make these singletons visible to Qt Creator's current code model (but not to other QML tooling)
• (Long-term) Converting this to QML_SINGLETON will make these singletons visible to all current QML tooling

Notes
Context properties are injected in the following files:

• src/common-lib/qml-utilities.cpp (dummy data)
• src/main-lib/main.cpp (StartupTimer)
• src/manager-lib/qmlinprocessruntime.cpp (ApplicationInterface)
• src/tools/appman/appman.cpp (build config)
• src/tools/launcher-qml/launcher-qml.cpp (StartupTimer + ApplicationInterface)