Feature request: Barriers layer(s)

[westnordost]

Barriers are currently omitted from the shortbread schema. At least point barriers such as bollards or gates are important to see on maps which roads are accessible.

There should probably be two layers:

1. point barriers: gate, bollard, lift_gate, block, cycle_barrier, swing_gate, stile, kissing_gate, turnstile, ...

2. line barriers: fence, wall, hedge, retaining_wall, man_made=embankment but also city_wall and natural=cliff

Regarding conflation, I think it makes sense to lump all those different gates into one (or if necessary have a second property tell them apart).