Improve connection handling

Connections that are not straight lines (the ones with connection anchors) need to have a different behaviour.
For a curved connection there are only two "curve points" specifying the curve and two endpoints of a connection, representing the source and the destination of a connection. By default, the curve points need to move when their corresponding endpoint moves by the same value on x- and y-axis.

The default behaviour of a polyline points should be to stay where they are when the endpoints are moved, unless the polyline points are explicitly selected.