feat: Component visibility (HasVisibility mixin)

[projectitis]

Description

This PR introduces a new HasVisibility mixin on Component. It prevents the renderTree method from progressing if the isVisible property is false. It therefore prevents the component and all it's children from rendering.

The purpose of this mixin is to allow showing and hiding a component without removing it from the tree.

An important note is that the component (and all it's children) will still be on the tree. They will continue to receive update events, and all other lifecycle events. If the user has implemented input such as tap events, or collision detection, or any other interactivity, this will continue to operate without affect (unless it relies on the render step - such as per-pixel collision detection).

This is expected behaviour. If it is not desired, the user needs to account for it (by checking isVisible in their code) or someone needs to create another mixin for HasEnabled or HasActive which would prevent these other actions 😁

I am very happy to make changes, take suggestions, etc!

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #2669

[projectitis]

Apologies for all the commits 🙄

[spydon]

Apologies for all the commits 🙄

Don't worry, they will all be squashed on merge

[spydon]

Lgtm, just one minor comment