Fix clipboard actions when navigating by subjects

When navigating by subjects and the clipboard actions don't work properly.
Steps to reproduce:

• log in as any type of user, go to "Browsing by subjects" choose a label which has at least one document.
• first use the select all checkbox and you see that it doesn't work properly, because the other buttons(copy, delete, paste, add content to worklist) do not become visible.
• second use the checkbox for a specific document and the butons (copy, delete, paste, add content to worklist) will appear, after this go to another label and the buttons will still be vizible and ready to be used.