Assistant: Load all applicationIcon pixmaps.

Qt Assistant, when used as a custom help viewer, would not load all
sizes of the application icon specified in the .qhcp file. Now
application icons look good everywhere, provided the files contain
multiple icons of various sizes.

Task-number: QTBUG-28267

Change-Id: I362e2feafa7944b0ad25627fb2fcea1b90af4e53
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>