Move target line out of viewport if no target set
Move target line out of viewport if no target set

If no target is set (present_value() or future_value() equals null), no target
line is visible as it is moved out of the viewport. If the target is set to
0, the target line is displayed as expected. Closes #1452.

quintel/etmodel,quintel/etmodel,quintel/etmodel,quintel/etmodel