refactoring flex.js for v/hflex with number
Merge branch '7.0' of github.com:zkoss/zk into 7.0
1. merge FixMinVflex and FixMinHflex into one function
2. fix bandpopup's flex with min issue
fine tune window, panel, and tabpanel
fine tune v/hflex with min
bug fixed for v/hflex issue
fix v/hflex issue
Merge branch '7.0' of https://github.com/zkoss/zk into update-combobutton
both the width of combobutton and popup become same, 
if prev one is large than last one.