Call add_child after set_rect to fix size bug

[rakkarage]

closes: #80234

[KoBeWi]

There is some odd preference for adding a child first and then initializing its properties (I tend to use it myself .-.), but recently I noticed that it affects sizing. I think we should re-evaluate this "convention". Even if the sizing problems are a bug, initializing stuff after adding it to tree is kinda backwards tbh (and in some cases probably less efficient).

[YuriSizov]

Even if the sizing problems are a bug, initializing stuff after adding it to tree is kinda backwards tbh

Well, it is possible that the context of the parent would affect how some things are resolved when it comes to sizing. Probably not with containers, but with anchors when setting a preset it may work differently, I think.

In this case, btw, I'm not even sure it makes sense to have a button control. We could just draw an icon and detect clicks over specific areas with a bit of lower-level code. That should avoid messing with control sizing all together. But that's not important for this PR.

[YuriSizov]

Could you please amend your commit message to follow the project's preferred style? The title of the PR is a good option.

[YuriSizov]

Thanks!