forward and backward coloring for custom overlay

[tiptoptom]

Use case
Coloring the left and right side of a street instead of it's center when the key contains :forward or :backward
E.g. for a custom maxspeed overlay

Why
Coloring custom overlay based on tags is nice, but creates too many colors when forward and backward is included in the keys.

Proposed Solution
When coloring key matches a key containing :forward or :backward, colors the left, right or both side(s) of the street instead (like the bicycle or sidewalk quest)

For my understanding, the solution should be almost identical to #403

[tiptoptom]

Unfortunately it is not that easy because it depends on whether there is left or right traffic.