(closes#4669)
Now instead of creating MultiLineString targets, we just create a bunch of
LineString targets. This makes the code simpler, and anyway the entity is
still there in `properties` for drawing code to decide what to do with the target.
Incidentally, this change allows iD to support an extrusion operation.
(Because each way segment has its own unique GeoJSON target now)
The goal here is that the code that draws the targets should know better
what parts of the lines/vertices are targetable, rather than just
relying on CSS to ignore the pointer events on the whole line.
e.g. when drawing a line, it's ok for it to loop back and connect
to itself, just not on a segment or vertex adjacent to the active
node.
These changes are needed now that `addNode`
* wants to preserve circularity
* automatically remove duplicates
* range checks index argument (can't call it with -1 anymore)
