Add opacity slider to categorized/classified vector layers
User story
As a user, I want to have the option of changing the opacity of my vector layer after categorizing or classifying it such that I can still view the map underneath.
Comments
Currently, this is only possible when displaying a vector layer with a single color. It should also be possible when using color bars in the modes "Categorized" and "Graduated", e.g. by adding an opacity slider as being used for raster data.