Add one-sided triangular markers to ways with sides (e.g. natural=cliff).

This generalizes the oneway arrow logic for adding SVG markers along a
line. Using that functionality, certain tags get arrows on their
right-hand side, indicating which side is "inside", e.g. the
right-side of a cliff is the lower side.

The list of tags considered to be sided (unless there's a
two_sided=yes tag) is:

- natural=cliff
- natural=coastline
- barrier=retaining_wall
- barrier=kerb
- barrier=guard_rail
- barrier=city_wall
- man_made=embankment

The triangles attempt to be reminiscent of the triangles used for
rendering cliffs on OSM (and elsewhere). The different tags get
different renderings (e.g. colors that match the main way, and
different spacings). In addition, natural=coastline is special-cased
to have blue markers (despite having a green way), to emphasise that
the "inside" of a coastline is the water.

Fixes https://github.com/openstreetmap/iD/issues/1475.

css/20_map.css

@@ -205,7 +205,8 @@ text {
 }
 
 .onewaygroup path.oneway,
-.viewfieldgroup path.viewfield {
+.viewfieldgroup path.viewfield,
+.sidedgroup path.sided {
     stroke-width: 6px;
 }