Added spacing between some lines in the main header of the scoreboard

As per 0004387, I touched up the main header of the scoreboard by adding a space between some of the lines, making it easier to read and improving the aesthetics. Here are some example images of how it looks now:

This required changing a few things:

1. I added an optional parameter CVar string chunks (for DrawString margin commands) to specify how many decimal places to show for float CVars. For example:

DrawString( value = "Damage factor is " + cvar( sv_coop_damagefactor, 2 ), textcolor = "Grey", bottompadding = 1 )

This always draws the value of sv_coop_damagefactor with two decimal places.

2. I separated the duel champion string and cooperative damage factor lines from SCOREBOARD_BuildLimitStrings, and by extension the "limitstrings" special value for the DrawString margin command. These were technically not "limit" strings. The champion string is now printed with the new championstring special value for DrawString.

3. DrawString margin commands that only contain whitespaces (e.g. line breaks) after they're refreshed are no longer shown (in multi-line blocks) on the scoreboard.