Feature request: write an uiBoxInsertAt function

[parro-it]

I noticed that tab widget has an useful insertAt method to add a child at a specified position.
A similar method is missing in box widget.
Since it already has a delete method, it would be a useful addition

It could be something similar to:

void uiBoxInsertAt(uiBox *b, uintmax_t before, uiControl *child, int stretchy);

It would be super useful because it allow one to replace a widget in a box with a different one, without resorting to remove and re-append all subsequent widgets in the box...

[andlabs]

Yes, all the containers need to have Append, InsertAt, and Delete on them; I just haven't gotten around to implementing them all yet.

[andlabs]

Now #309.