OWMosaicDisplay: Remove Class dist if no class_var
OWMosaicDisplay: Remove Class dist if no class_var

Remove Class distribution option from Color comboBox, when dataset has no class variable.

marinkaz/orange3,cheral/orange3,qPCR4vir/orange3,marinkaz/orange3,marinkaz/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,marinkaz/orange3,qPCR4vir/orange3,qPCR4vir/orange3,kwikadi/orange3,marinkaz/orange3,kwikadi/orange3,kwikadi/orange3,cheral/orange3,kwikadi/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,marinkaz/orange3,cheral/orange3