feat(dia.ElementView): add getTargetParentView() method

[kumilingus]

Description

dia.ElementView.prototype.getTargetParentView(evt: dia.Event): dia.CellView | null;

The method returns a cell view (if any) that would become the parent of the currently dragged element view if the dragging were to finish immediately.

Usage

paper.on('element:pointermove', (elementView, evt) => {
    console.log(elementView.getTargetParentView(evt));
});

// It can also be used while the user is dragging an element from the stencil
stencil.on('element:drag', (cloneView, evt) => {
    console.log(cloneView.getTargetParentView(evt));
});