Draft: implementation of "implements" arrow for interfaces

Cause: The implementation of the interface checkbox

With said implementation, we now have another, previously not intended, type of element for the umlPuzzle: The interface. The difference between the interface and the regular (even abstract) classes is, amongst other things, that the inheritance has another meaning in between it and other elements. This difference is not a semantic but also stylistic one, as the inheritance arrow means "implements" instead and has gotten a dashed line attached to it as opposed to straight one.

Since we can't (for time constraints) implement another, totally different, type of element and the only difference between "implement" arrows and "inherits" arrows is the dashed line, here an example implementation for an automatic detection as to what element is to be used in the relation is made.

Known issues:

When changes are made to the interface / class transforming one into another, the already drawn arrow will not automatically be changed too and thus will display a false relation type! It only works, when the change is made before creating the relation, then it is automatically and correctly detected.