Provide control over feature override priority

[pmconne]

As discussed in #7430.
If, for example, the color is overridden for both a model X and an element Y belonging to model X, the existing rules say that the element draws using the model's color override.
The only way to make the element color take precedence would be to not override the model, and instead override every single element in model X except element Y to be one color, and element Y to be another color.
That is unnecessarily expensive.

We should provide a way to specify that an element override should take precedence over a model override when they conflict.
Consider what similar scenarios should be addressed (e.g., conflicts between subcategories and elements or models).