Make children life update in BDL-load

[smoogipoo]

The issue that sparked this: ppy/osu#2496

Explanation:
DrawableMedal binds to medalContainer.OnLoadComplete, to set the position of some text contained within that container (i.e. unlocked.Position = new Vector2(0f, medalContainer.DrawSize.Y / 2 + 10);).

This is expecting two things:

1. The children of medalContainer are loaded.
2. The children of medalContainer is alive.

(1) Is fulfilled by LoadComplete being called from UpdateSubTree, which occurs post-all-children-being-loaded.
(2) Is where the issue occurs. CompositeDrawable.load() called UpdateChildrenLife, but it would get blocked by load state check in checkChildLife: https://github.com/ppy/osu-framework/compare/master...smoogipoo:async-child-life?expand=1#diff-2f079688019880634dc5249946a02502L407.
The next time child life gets updated is post-LoadComplete. So the children of medalContainer were loaded, but not alive.
This results in autosize not finding alive children and giving a DrawSize of 0.

A similar change as proposed in this PR was submitted some ~6 months ago (#1072) but then reverted (#1083). The reasoning was that some elements were broken (https://streamable.com/6pwqf, ppy/osu#1366). Although these elements are not broken with the same changes as in the original PR, I chose to go down a different route and maintain that "children are not alive or loaded before their parent is loaded".

There are a few significant changes here:

1. checkChildLife no longer does load state checking. This is now done in UpdateChildrenLife and AddInternal - the only other use-case of checkChildLife through publicly-acccessible methods.
2. The previous functionality of UpdateChildrenLife has been moved to a new private method checkChildrenLife. The new method, like checkChildLife, does not perform load state checking.
3. BDL-load now uses checkChildLife, bypassing the load state checking of UpdateChildrenLife.

Additionally, I think the following will result in some discussion:
I've moved the call to check children life to LoadAsyncComplete, which happens after BDL-load,
but still on the asynchronous context, and prior to the drawable being given a Ready load state.
This was done because the alive state of children defined in BDL-load would previously be computed on the Update thread, when UpdateChildrenLife would be normally invoked.

Testing

It's hard to define exactly what to test with this. Obviously I've tested that it fixes the issue that sparked this, but I've also gone through all visual tests (osu! and framework-side) and found nothing breaking.

[smoogipoo]

Made a flow chart of the loading procedure after this change, probably to be used in the wiki.

[smoogipoo]

Updated flow chart.

Raw for future reference:
osu! load sequence-4-3.xml.zip

[smoogipoo]

Raw: osu! load sequence-4-10.xml.zip

[peppy]

Simplified diagram on the wiki: https://github.com/ppy/osu-framework/wiki/Diagram:-Loading-of-a-drawable