[DMN Designer] Different icons for different connector types

The toolbox shows icons for the different connection types available from a given node.

At the moment they are all the same icon and it is difficult to know which toolbox icon represents which connection type. Until https://issues.jboss.org/browse/DROOLS-1944 a short-term fix is to simply ensure each connection type has an icon more correctly representing its type.

Manual Acceptance test

Each node type has appropriate connector icons in the context menu