Improve cache

[JiuqingSong]

This is an improvement to existing cache code.

1. Rename function updateCachedSelection to updateCache to let it update both cached content model and selection
2. In setContentModel, we need to flush mutations before updating cache, so that the previous mutations will not impact the cached model, since we know the passed in model is the latest one
3. Do not set DOM selection if the new selection is the same with the old one
4. Fully decouple TextMutationObserver and DomIndexer. Now TextMutationObserver only capture mutations, and let CachePlugin.onMutation decide how to handle the mutations
5. Refactor the callback onMuation of TextMutationObserver to let it handle all kinds of mutation, with a new comibined parameter to specify mutation types and content
6. Add/update test cases