Selector detection change

[afonsolage]

Fixes #39

Changed

• Selectors will now detect changes on entities and apply the stylesheet;

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Overall I have to track which SelectorElement matched which Entity, so we can convert the given SelectorElement into a Query<T> and use bevy_ecs to keep track of changes. When some Entity is changed, the whole StyleSheet will be applied, since fine grain control over will be very complex and I don't think it'll worth it at the end, since applying a StyleSheet is cheap (even tho there are plenty of optimization left to do).

StyleSheetState now has another member, for each Asset<StyleSheetAsset>. The new TrackedEntities member is populated when system::select_entities in ran, that way, we don't have to traverse the entity tree twice.

TODO:

• [x] Find a way to get a Query<T> from a SelectorElement inside the watch_tracked_entities system;
• [x] Write some examples;
• [x] Update docs and changelog;

[afonsolage]

Most of the work is done. I just need to fix a bug where the new system which watches for changes, is reacting to changes made by the bevy_ecss itself.

[afonsolage]

I was able to fix the change detection, but after more tests, the Interaction component change is not being detected. I'll do more research on that.

[afonsolage]

Ok, everything seems to be working fine now. Let's wrap things up and prepare to merge this. I'll write a new interactive example and write docs and changelog.