Fixes bug #990: SC.ListItemView Doesn’t Render Right Icons of the View’s rightIcon Property

The code for handling the rightIcon property was missing in the render method. It has been added including all needed unit tests.
Additionally, the first part of the previous code has been adapted to the code for the (left) icon: In case of contentRightIconKey, it is only rendered if a value exits for that key (fixes a bug not opened yet).
See bug #990.