Drag operation prevention

[yanisslav]

Currently, we have a cancelDrag method that allows us to cancel the dragging operation.

While this method is helpful, we need to add a mechanism that allows us to prevent the drag operation at all, similar to how preventing the HTML5 dragstart event prevents the dragging operations. See: https://dojo.telerik.com/emIcUjUv/3.

cc: @telerik/kendo-draggable-reviewers