SVG overlay: ctrl+alt+click inserts node along skeleton

This adds the ability to ctrl+alt+click in empty space
(command+alt+click on Mac) when in skeleton tracing to insert a new node
along the edge of the active skeleton nearest the cursor. Closes
acardona/CATMAID#684.
SVG overlay: prevent inserted node from stacking

Add an exlcusion radius around existing nodes so that nodes inserted
with alt+click along edges are less likely to overlap an existing node.
SVG overlay: clamp inserted nodes to slice z

When inserting a node along a skeleton with ctrl+alt+click, clamp the z
value of the inserted node to be on the active slice.
SVG overlay: comment node insertion (minor)
Merge pull request #737 from aschampion/issues/684-node-insertion-on-edge

Add node between existing nodes on the active skeleton with ctrl-alt-click