Replaces .color property of Labels layer by assigning colormap with DirectLabelColormap to match napari 0.5.0

An Attribution Error raises if the viewer is closed from code because somehow an Image layer can be present at self.layer_select.value, which has no 'properties' property

This reverts opacity attribution to the layer (the value of the combobox), not to the combobox (which is 'layer_select')

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This reverts commit be58dc0.