Using groups in OSX seems to add the top of the window until resize

If I use groups it looks fine in linux but in OSX a bunch of padding gets added to the top of the window. Looks to be the height of the combined children. As soon as the window is resized this magically fixes itself.

Rough demo program:

```
package main

import (
"github.com/andlabs/ui"
)

func main(){
ui.Main(func(){
window := ui.NewWindow("boo", 100, 30, false)
vbox := ui.NewVerticalBox()
vbox.SetPadded(true)

for i := 0; i < 10; i ++ {
grp := ui.NewGroup("Group")
grp.SetChild(ui.NewButton("THING"))
vbox.Append(grp,true)
}
window.SetChild(vbox)
window.Show()
})
}
```