override append on containers to add child properties

[nzjrs]

Because {H,V}Box have flex set, we can override append to accept expand and fill properties, which use the flex child properties

https://css-tricks.com/snippets/css/a-guide-to-flexbox/

should probably set align-items = centre on the containers too.

can also probbably set width='inherit' if there is only one child and this is a vbox