morphic: disable dropping into CanvasMorph
morphic: disable dropping into CanvasMorph

... because children are not shown inside <canvas> and it is highly
disturbing to have dropped morphs "disappear".

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