* UI layout v2.
All controls now act like containers.
Moved anchor/margin layout model to LayoutContainer
Implement basic WPF-like attached properties for the above LayoutContainer.
* Fix VV.
Give BoxContainer a preset capacity to reduce allocations in layout.
Removed Control.GetChild(string) and similar. This was no longer used (and was a pretty bad idea in general), and the implementation had a lot of wasted book keeping to do.
Use UpdateLayout() instead of DoUpdateLayout() in Control minsize changed to avoid doing redundant layout, and by extension style & minsize, updates.
Used RemoveAllChildren instead of DisposeAllChildren in EntitySpawnWindow because it's faster (way less stuff to clear, just let the GC handle it).
Optimized ResourcePath with a better GetHashCode(), better Equals, replacement for List<T>.ToArray()
Fixed enumerator allocations in stylesheet SelectorElement matching to speed up style updates.
ResourceCache now caches fallbacks to avoid constructing Resource instance when fetching fallback explicitly.
ResourceCache now has nested dictionaries for Type -> ResourcePath -> BaseResource storage of resources.
Use calculated property instead of auto property for TextureResource.Fallback
* Refactor UI system.
Deferred updating is used for styling & layout. This fixes the awful time complexity of containers.
Removed SetDefaults and Initialize. They were a bad idea alright.
* Fix build on .NET Framework.
