Drag & Drop line drawing.
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.FreeTransform control panels around.