fix layoutregion's splitter would change _open's value when clicked
splitter-button-disabled
keep open unchanged when it's not collapsible
Merge branch '7.0' of https://github.com/zkoss/zk into fix-layoutregion-splitter
Merge branch '7.0' of https://github.com/zkoss/zk into fix-layoutregion-splitter
minor fix LayoutRegion's doClick function