Distribute extra empty space among gaps between offsets, xdisp, and adisp

On Fedora 28 with dark Adwaita theme, there is no spacing between the offsets and the hex display. If the window is enlarged horizontally, the extra space is used only for the gap between hex and ASCII display. In addition, when the window is in focus, the font intensity/brightness difference between the displays is diminished to the point where it is rather challenging to visually distinguish between the offset and the first column of hex display.

This PR attempts to mitigate the issue by distributing the extra space among the gap between offsets and hex display and the gap between hex and ASCII displays. If offsets are not displayed, all extra space goes to the gap between hex and ASCII displays, just as before.

Before:

After: