Ensure a space of 0.5em between dt and dd in legend

In font Verdana the text "(+-123)" has a width of more than 3.5em.
With dd.mw-changeslist-legend-plusminus { margin-left: 3.5em; }
there is no space between "(+-123)" and the description.

Change-Id: Id8e4bc478cb00eaa9271c5a6cb580fd042d1c026
Merge "Ensure a space of 0.5em between dt and dd in legend"