Scrollspy acting up when targets are hidden

If elements are hidden using `display: none;` which are targets mapped by scrollspy, the plugin might select an incorrect or no menu item.

Checking if the target is visible solved this.
fixed .clearfix example, + doc link
Merge pull request #12035 from tomByrer/master

fixed .clearfix example, + doc link
drop the title
fixes #10658 jQuery Popover content loses bound events on second setContent call.
Merge pull request #11203 from jasny/patch-1

Scrollspy acting up when targets are hidden