AddTactilePavingCrosswalk and AddTactilePavingKerb should interact better

[jyasskin]

Use case

In an intersection like https://www.openstreetmap.org/node/40538731, the app will ask about tactile paving 12 times when there are only 8 places (the AddTactilePavingKerb quests) that could have tactile paving or not. The highway=crossing nodes can be inferred from the kerb tags, and a yes answer to AddTactilePavingCrosswalk answers 2 kerb nodes at once. It would be nice if the app optimized this interaction a bit.

This was discussed some in #2347, but here I'm suggesting an improvement to how we live with the double-tagging that the community seems to have settled on.

Proposed Solution

1. In AddTactilePavingKerb.applyAnswerTo(), we should look for the next nodes in the adjacent footway=crossing way, and if those are a highway=crossing node and then a barrier=kerb; tactile_paving=* node, we should set the crossing node's tactile_paving key to the appropriate value.

2. In AddTactilePavingCrosswalk.applyAnswerTo(true), we should look for a unique footway=crossing way that contains the highway=crossing node, step out to find its kerbs, and set them to tactile_paving=yes. In AddTactilePavingCrosswalk.applyAnswerTo(false), if both kerbs are set to tactile_paving=yes, something's out of date, so it'd be nice to flag both kerbs to be re-quested, but it's not obvious how to do that.
   I think the highway=traffic_signals; crossing=traffic_signals case is irrelevant here, because those only happen at the center of an intersection without separated sidewalks, so there won't be any kerb nodes to modify.

[westnordost]

This would probably make sense. However, this is not possible in applyAnswerTo, that is not how the function works or should work. A quest answer is always only applied to one element. This is a limitation of the app, but I am happy about that limitation, I need to limit the complexity of the app somehow.

However, this may be a good validation check for a validator. See if tactile_paving differs for the crossing node (if it is set) with the tagging for the kerbs.