Node overlay: increase mouse area for connector arrows

Add a larger transparent line to each connector arrowline for catching
mouse events. The duplicate element appraoch was taken because the
transparent stroke trick for nodes does not work for lines (since there
is no fillable area). The CATCH_SCALE value (currently 3) controls the
width of the mousecatching area relative to the scaled EDGE_WIDTH of the
connector. A single, fillable rectangle could replace both elements, but
rectangles distractingly do not scale until node update. This overhead
for two elements has less impact for connector arrows as they are less
numerous than tree edges and nodes.

In SVG2 the duplicate transparent line could be replaced with a
marker-segment element shared by all connector arrows, but this feature
does not yet have wide browser support.

Closes acardona/CATMAID#858.