Do not allow to use the same measure for both Area and Symbol Layer

[bprusinowski]

Is your feature request related to a problem? Please describe. Right now users can choose the same measure to color shapes (areas) and coordinates (symbols). This doesn't make a lot of sense, as it's a duplication of the same information – so it would make sense to disable such possibility.

Describe the solution you'd like It shouldn't be possible to select the same measure for Area Layer color & Symbol Layer size. Probably it also makes sense to extend this also to newly introduced Symbol Layer colors.