Support for QML Singletons

[AndreSomers]

With the phasing-out of context properties and the promotion of using QML Singletons instead for interfacing between QML and C++, it would be nice to get a view of the available QML singletons and introspect their properties, signals and methods.

[AndreSomers]

Note: I know the singletons appear as instances in the object tree, but those may not all be all that easy to locate. Having them somewhere in the QML area would be nicer, just like you have the Qml context available.