Make using of singelton types less error prone

QML offers solution for making QML elements singletons, but current Qt Wayland API does not make elements that are singletons from the protocols point of view into QML singleton types. The goal of this task is to investigate if using non-singleton QML types improves ease of use of API in writing compositor or should types that are singletons in protocol be also singletons in QML. If we choose to keep the current non-singeleton QML types, then we should at least add reference counting and print a warning if more then 1 object is created of type that should be singleton.

If using QML types produces less convenient APIs, perhaps something can be done to improve QML singleton support in qtdeclarative module.

With current APIs a user can start created many instances of, for example, TextInputManager {}, which would not make sense.