add highway=construction

[der-stefan]

Streets under construction should not disappear, but be shown e.g. in gray.

[max-dn]

highway=construction in gray looks like that: that area with all highways under construction Or that: that area with really constructed highway

I got problems with casing... small highways with black casing look like railways. So I did just small casings for bridges.

In osm carto they take the same color for highway=X and construction=X. Maybe it's a good idea, maybe it looks too much like a candy cane for this style...

[der-stefan]

Yeah, looks better than nothing! Let's try it out.