Investigate on a way to order styles

[werediver]

The current implementation applies the styles in the order of definition. This is prone to errors (e.g. a style for a base class can override a style for a subclass, if the former is defined after the latter).

Investigate on a clean and reliable way to order styles depending on the subtype relationships of their targets and markers.

If a clean and reliable way to order styles is found, plan transition to a flat style list instead of the current tree-like structure (in a separate ticket).