Different color for stairs that go to another dungeon branch.
Adds four new glyphs, S_br{up,dn}{stair,ladder}, which use the
same character as normal stairs/ladders, but yellow color.
In tiles, the up/down arrow is yellow-green instead of while-blue.
This feature has been around a lot and is in several different
variants, but this is implemented from scratch so tiles work too.
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