Rethink LayerAppearance

[KaiVolland]

We took this concept from OL3, but in practice I discovered that there might be nearly no case we're an LayerAppearance would be shared by multiple Layers.

For Example:
If you share an appearance with two layers. The opacity and the visible are shared. So if you make one layer initialy invisible. The other layer will be invisbile to. Same for opacity.

In my opinion we should at least move those to properties to the Layer Model.

attribution, minResoltion, maxResoltion might be shared sometimes.

Feedback would be cool, as i need clarification on this topic soon. 👯
@marcjansen @buehner @weskamm @dnlkoch

I would then takle this asap.