🐛 [ComboBox] handle ComboBoxItem.child height #1230

If ComboBoxItem.child contains a Widget that takes more than the default one-row height, it overflows.
So, for instance, if we want to add a caption or subtitle in each ComboBoxItem, we currently can't.

This is what it may look like

It's another subject, but it could be handy to handle separators and grouping like this also :