Fix double-click paste error

When double-clicking on an item to paste it, the ui shows two pasted
items, but after a page refresh there's only one item pasted.
This fix avoid prevent double clicks.
Merge branch 'mlocati:fix-doubleclick-paste-2'

github.com:mlocati/concrete5 into test_967_double_click_clip

Conflicts:
	web/concrete/elements/scrapbook_lists.php