css: Add GtkCssSelectorTree creation and matching
From a set of GtkCssSelectors and the rulesets they match to we create a large decision tree that lets us efficitently match against all the rules and return the set of matched rulesets. The tree is created such that at each level we pick the initial rule in all the considered selectors for that level and use put the one that is in most selectors in the node. All selectors matching that are put in the previous part of the tree.
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