Disallow dragging fragments above a super call

[Matthias Schoettle]

When moving fragments within a message view (see #536) certain bounds are checked to ensure that a fragment can not be moved to an invalid place.

It is still possible to move a fragment above a super constructor call.

Steps to reproduce

1. Create class Super with sub-class Sub
2. Add a constructor to each class
3. Create a message view for the constructor of Sub
4. Add the super constructor call
5. Add another fragment after the super constructor call
6. Drag the other fragment on top of the super constructor call

Expected Result

It should not be possible to move any fragment above the super constructor cal.

See

MessageViewUtil.findLowerBound method.