incremental restyle: Introduce StylingMode and deprecate explicit dirtiness
This is another chunk of work to move us toward the new incremental restyle architecture.
Eventually, we'll make a fine-grained decision at each node about what style to recompute based on the RestyleHint on the node data (along with other things). For now, we use the existence of RestyleData as a coarse-grained approximation of this.
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c8b6ece97b
