Recompute slider range for out-of-range values (issue #262)

The maximum value of sliders will now be recomputed when the user enters a larger value in the LineEdit.
I was worried about users accidentally typing in big values and making the slider range too big so I made the maximum value shrink if the entered value is <= 1/3 the current max.
I don't know how you feel about this. Is there some other way you would prefer to handle it?