Clicking in space when a connector is selected adds the presynaptic node, which is sent to the "Isolated synaptic terminals" group.
Clicking in space when a connector is selected adds the presynaptic node,
which is sent to the "Isolated synaptic terminals" group.

This facilitates the work flow with shift+alt to create connectors
for which the selected node is postsynaptic, and also ensures
that the isolated nodes created exclusively to be presynaptic placeholders
to the connector are stashed away under the "Isolated synaptic terminals"
group rather than left clutering the user's staging folder.

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