fix(layout): crop connection on first intersection

Related to bpmn-io/bpmn-js#48
fix(core): correctly wire parent -> child relationship on add

This commit fixes a bug that caused an element to appear twice in its
parent once it was added to the canvas.
feat(modeling): move child elements with shapes

This commit adds the ability to move child elements with a shape.

Related to bpmn-io/bpmn-js#125