fix onrender hook for nested widgets

[Swatinem]

As I thought in #91, there indeed was a bug that onrender was called before a widget was mounted for nested widgets.

I also added a failing test because dynamic widgets are mounted at the wrong position. That is also something that should be fixed by separating create and mount.

[codecov-io]

Current coverage is 90.76% (diff: 100%)

Merging #103 into master will not change coverage

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[Rich-Harris]

Yeah, that makes a lot more sense 😆 I'll change _parent to root since that's a more accurate description of what it is (we do use root elsewhere to mean data or state or context, which is pointlessly confusing – should probably unify those)