Improve speed of css selector lookup for large stylesheets

[Oudwins]

Currently to check if a selector exists we look at each element. This may be slow for large stylesheets

There are a few ways we could improve the lookup speed but I have not yet decide on the best approach:

1. Some how sorting the rules array on each insert allowing a binary search to be performed for each lookup
   This would require a way to sort any css selector while keeping all media queries at the bottom of the array (this is a must)

2. Since css selectors in Dstyler must be unique on each scope. We may keep a selector->ast_rule_node hashmap for O(1) lookups

Both options have their tradeoffs