NPC hover-label is displayed above UI panels (eg. inventory)

[M.Sz.]

Describe the bug
If NPC walks under the inventory window, we still can see his label

To Reproduce

1. Go to inventory window
2. If your mouse will be over the NPC, which walks under the menu, you can see its label

Additional context
That's minor bug

[Thomas Christlieb]

This problem only existed for the inventory panel. Skilltree, questlog and heroStats Panel are rendered differently and don't have this problem.