BUG: multicolumn PopupMenu layout issue (PR included)

I think there’s a typo in PopupMenu::workOutBestSize() method that makes a multicolumn PopupMenu layout like this (column widths aren’t equal):
instead of this:

I guess the bug only manifests itself when using PopupMenu::Options::withMinimumWidth() to set a larger than default width.

Here’s the pull request: