Fixed|Refactor|Resources|Renderer: Removing assumptions about submodel count

In many places the code makes hidden assumptions about the number of
submodels in the defined 3D models. These changes add additional
checks to ensure that no out-of-bounds access occurs.

Now also checking the DED models' submodel array and not just the
renderer's model data.
Client|UI|ScrollAreaWidget: Reduced page up/down step size to half the visible size

Slightly more comfortable to use than full page jumps.