Drag & Drop line drawing.

### How to

The idea is to draw a temporary SVG path while the user is creating the line by drag&drop. When the user releases the mouse cursor when in the line drawing mode (`creatingLine === true`), we create the JointJS `basic.Path` shape based on the line coordinates gathered during the drawing. The result of this is a normal JointJS shapes that we can put the `ui.Halo` and/or `ui.FreeTransform` control panels around.