More detailed documentation of addEmptyObject

The documentation explains the very basics of how to add an empty object to openflipper, but it does not mention the signal slot structure, that is involved with it.

Explaining how addEmptyObject is processed in the documentation would help to avoid out of order execution. Maybe in combination with a short example similar to the one in https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/issues/108