Add UI for configurating attributes required for certain features when creating them

[kb173]

Currently, creating a new wind turbine is limited: its height and diameter are at "0" since these attributes are not set anywhere. We need an UI for configurating such values depending on the RenderInfo.

We can get these attribute names from the RenderInfo classes: for example, the WindTurbineRenderInfo has the required height_attribute_name and diameter_attribute_name. However, these are technically identical to other variables such as the ground_height_layer, so we need some way of differentiating attributes from other things. (All variables do end in _attribute_name, but that seems like a very volatile way of checking for them...)

[MathiasBaumgartinger]

Heavily related with #249

[kb173]

"creating a feature" here refers to clicking into the landscape / minimap and instantiating a new object (or other feature) within the currently selected GeoLayer

[kb173]

The implementation for #333 should go hand in hand with this