DataMapper: Use a smaller object when dragging

[lordrip]

Please describe the feature that you want to propose

When dragging a field from one schema to another, the object being grabbed is the same size as the original row, making it a bit complicated to manipulate it.

The goal of this ticket is when grabbing the field, the object should be smaller, so it's centered on the user's cursor.

[igarashitm]

First I thought we can narrow the draggable area horizontally, probably limit to the left icons and field label. However, it then also force users to move the mouse further away to the icons and labels when the screen is wide enough so the field is wide.

Ideally we want to keep the draggable area as wide as currently is, but when it starts dragging, the field label jumps and is shown right under the mouse cursor as well as the dropping area evaluation happens right on it.