ColumnMenus: if open second menu without closing first, get duplicate items

[PaulL1]

Open the first menu, then open the second (or last) without closing the first.
http://plnkr.co/edit/mtEO8fkK6ohqQDpgbyGE?p=preview

[PaulL1]

Tracked to #2845, so have reverted that. But that functionality is wanted, so we need to fix and then re-merge.

[PaulL1]

Refer #3049, which is the PR for this. Waiting on some minor documentation updates then it can be merged.

[PaulL1]

Closing, track via PR.