Connect postsynaptic treenodes with connectors

When a treenode is activated, alt-shift clicking on an existing
connector creates a link with the node postsynaptic to the connector.
This allows interface parity with creating a new connector to which
the activated node is postsynaptic by alt-shift clicking in empty
areas. Closes acardona/CATMAID#712.
Merge pull request #716 from aschampion/issues/712-presynaptic-connector-assignment

Connect postsynaptic treenodes with existing connectors via alt-shift click