Rounding errors using numpad hotkeys

[lukmuk]

When using Numpad 4 and 6 to change scale-bar size rounding errors appear for scale bar lengths < 1.
As a result, 1 µm looks the same as 0.5 µm, since the latter is rounded to 1 for the scale-bar display.
Scale-bar lengths below 0.5 show "0" as scale-bar length.