Some minor cleaning up.
SingleCellSimulation plugin: made model parameters more obvious (closes #169).

As in COR, we use a colour coding to distinguish between the different
types of model parameters:
 - Constants (editable): filled red circle with a white hole;
 - Computed constants (non-editable) filled red circle;
 - State (editable): filled blue circle with a while hole;
 - Rate (non-editable): filled blue circle; and
 - Algebraic (non-editable): filled green circle.