validator should accept some kinds of way overlaps

[A_Pirard]

A convenient and sensible way to map ways is to

• map the waterways in one thread at layer -2
• overlap culverts and bridges at layer -1
• map roads in one thread at layer 0
• overlap road/railway tunnels at level 1

convenient: obviously easier not to have to tag many way segments all the same
sensible: for example,

• a tunnel is not the river, it's something around it
• a bridge is concrete under continued tarmac

in fact, the way runs uninterrupted.

Hence, JOSM should allow such overlap combinations and forbid others.
Or, even more simply, it should allow them because they're not at the same level=layer.

Note that it should be supported to map an overlapping segment even of the same kind and layer to add or modify a tag on a short distance. Say turn a tarmac road to cobblestone for 100m with overlap=cobblestone.