[BUGFIX] Fix ContentObject Label and Icon

Add a check if the content object label exists. If there
is no valid label than a help message is displayed. Furthermore
fix the icon handling and move the handling of the right
icon file to the prepare function of the Loader.

Resolves: #60811
Change-Id: I0b85ef50557e167d095a06952d076fe7ade10c12
Reviewed-on: http://review.typo3.org/32018
Reviewed-by: Tim Lochmüller <tim@fruit-lab.de>
Tested-by: Tim Lochmüller <tim@fruit-lab.de>