Please review fix for cr6806861
Description: copy-paste worked incorrectly for trees in config editor (question tree and TreeQuestions e.g 'Tests to Run').
Default swing copy-paste mechanism uses toString() method from the object containing in model (Question for PathPanel), but I've decided that replacing TransferHandler would be better than implementing fake toString() for Question
> Then just evaluation which question types have a rendering which doesn't copy-paste well. Tests 2 run it seems, does the PropertiesQuestion table work?
I suppose that such problems (copying class name e.g TreeQuestionRenderer$TQNode@b084a6) have only Tree-containing question types. PropertiesQuestion supports copying only from editing mode at the moment but it is easy to implement copying for it. Question types as FileList may also have similar bugs but I haven't seen them