refactor!: Separate ComponentSet from the Component

[st-pasha]

Description

Currently, ComponentSet is tightly coupled with the Component class and is partially responsible for handling the Component's lifecycle.

This PR breaks the two classes apart: Component will now perform its own lifecycle management, and ComponentSet will focus on being a simple container.

Several optional breaking changes introduced here. "Optional" because they can be reverted, at the cost of making the code less nice. However, since ComponentSet is mostly internal class, I deem the risk to end users low:

• Component.reAddChildren() was made private. Calling it by end users was dangerous.
• Component.prepare() declared protected. Likewise, it was never intended to be user-callable.
• ComponentSet no longer has field parent, as it is no longer needed.
• ComponentSet constructor no longer has argument Component parent, same for ComponentSet.createDefault.
• Since we're changing ComponentSet's constructor anyway, I use this opportunity to also change the comparator into a named argument, which allows it to be optional, so that no-argument ComponentSet() would create the component set with default comparator function. This also allows ComponentSet.createDefault to be greatly simplified, and in fact we will be able to remove it in Dart 2.15.
• ComponentSet.addChildren() removed.
• ComponentSet.add() and ComponentSet.addAll() are now prohibited (thus, the user can no longer say myComponent.children.add(newComponent)). In fact, it would be nice to prohibit direct manipulation of ComponentSet by the user altogether, but this PR doesn't go that far.

Out of scope for this PR: methods for removal of components.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• (NA) I have updated/added relevant examples in examples.

Breaking Change

• Yes, this is a breaking change. (Indicate it in the Conventional Commit prefix with a !,
  e.g. feat!:, fix!:).

[spydon]

Lgtm!

[luanpotter]

Is this needed/used anywhere? The reason to have it before was bc it was actually configuring the created set. Now you could just call ComponentSet() directly?

[st-pasha]

Yes, this is used by Component.childrenFactory. Somehow using a simple function comprehension () => ComponentSet() produces an analyzer warning there.

[st-pasha]

Apparently, they fixed the warning just 8 days ago: dart-lang/linter#3118, but it will probably take time until we can actually use that.