Selection Color should allow opacity value

[neumann]

One can define a selection color (e.g. yellow), but the selected color is always fully opaque. It would be good if one could specify an alpha/transparency/opacity value along with the selection color.

Often one wants other layers to shine through while working with selected feature geometries.

The identify tool uses opacity, but it is not configurable.

Ideally, both the identify and the selection colors should allow custom opacity values.

Thank you for having a look at this small enhancement!