Don't depend on Qt private APIs

[plfiorini]

Right now the code heavily depends on Qt private APIs, both in the compositor and QPA code. Here's what we need to do to remove this dependency:

• [x] Remove QObjectPrivate and replace the d pointer with a QScopedPointer
• [x] WaylandMouseTrackerPrivate no longer inherits from QQuickItemPrivate
• [x] WaylandQuickItemPrivate no longer inherits from QQuickItemPrivate
• [x] Copy code from QQuickItemPrivate::paintOrderChildItems() to be used by WaylandQuickOutput
• [x] Replace QFactoryLoader with a custom implementation to load our plugins
• [x] Remove QtCore private API usage from AuroraEdidSupport
• [ ] Move Aurora::Compositor::Internal::WindowSystemEventHandler to the QPA plugin