Reimplement FindChild and add FindChildrenStartWith

[SadPencil]

The original FindChild<T> is implemented in a less extendable way. It is nicely reimplemented now and a new function named FindChildrenStartWith is added.

I am working on a feature that allow mod developpers customize buttons with their own tags, therefore I need to match these buttons with prefix name but the current source code does not provide such a function.

[github-actions[bot]]

Nightly build for this pull request:

• artifacts.zip This comment is automatic and is meant to allow guests to get latest automatic builds without registering. It is updated on every successful build.