Keep track of intersecting segments

Because we have nodes sorted by segment distance from the
editing vertex in #handleMouseAtPixel(), it is cheap to create a
hash of intersecting segments there. Now in #handleDragStart(),
we do not need to measure the distance of the vertex to the
segment. Instead, we just test if the segment is in the hash.
Added another linestring to show shared segment editing
Limit shared segment editing

To avoid surprises, we enable shared segment editing only on
segments that have the same vertex coordinates.
Add a layer with real data to the example
Merge pull request #1807 from ahocevar/float-no-zero

Modify interaction sometimes does not allow to create a new vertex