Unify relationship creation in structural view

[Matthias Schoettle]

Right now, there are two different ways for creating associations (see #346):

1. Select class, double-tap on other class
2. draw line from beside class onto another class (unistroke)

The former also works for creating inheritance by performing a tap-and-hold gesture instead.

I suggest to change it to the following:

Always a line has to be created. After the gesture is performed, present a popup that asks which kind of relationship should be created (choice depending on possibility). This would be consistent with the way constraints are created in the feature model and would make code for classifiers easier (because selection does not have to be taken into account anymore and is only used to move several classes).

[Matthias Schoettle]

Resolves #381: Unifies relationship creation.

Relationships are only created by drawing a line on top of the "to" class. A selector shows the possible options, i.e., only the possobile options are presented to the user.

→ <<cset d10e397f1e48>>