multiple bugfixes for tree widget
multiple bugfixes for tree widget

pascience/LivelyKernel,pascience/LivelyKernel,NikolaySuslov/LivelyKernel,LivelyKernel/LivelyKernel,NikolaySuslov/LivelyKernel,NikolaySuslov/LivelyKernel,LivelyKernel/LivelyKernel,LivelyKernel/LivelyKernel,pascience/LivelyKernel