Inferred values for autofilters

[derickson.e@…]

I believe autofilters would be more useful if some tags were inferred when not actually present. For objects missing a layer=* tag, I propose the following scheme:

• bridge=*, power=1, location=overhead: assume layer=1 if not specified
• tunnel=yes, tunnel=culvert, location=underground: assume layer=-1
• tunnel=building_passage: assume layer=0
• Other highway=*, waterway=*, railway=*: assume layer=0

A user who had autofilters enabled for the layer tag could then use autofilters as if those tags were present. For example, it is not recommended to tag layer=0, but such a filter would be useful when working on a complex motorway interchange, filtering out all objects not assumed to be at surface level. The autofilter buttons would still only appear when an object with a layer=* tag is in the viewport, as is currently, but whenever an autofilter button appears, there would also be a "0" button between the "1" and "-1" buttons.