Don't create association in opposite direction

[Matthias Schoettle]

1. Select an implementation class
2. Double-tap on a class

Result: An association from the class to the implementation class will be created

Expected Result: Nothing, because an association from an implementation class to a class is not possible.

[Cécile Camillieri]

Resolves #346, #353: association with unistroke gesture works with implementation classes and only creates unidirectional association. Association in the opposite directions not created anymore.

→ <<cset b73069b282ef>>