Moving a field over an entity border creates the field in the component

Fields are not allowed to be moved to the component. A popup with the text
"Unable to move" is presented when we try to drop a field in the component
using the Paint Tableau.

However, if we paint an entity in the component and give it a border, we can
then move the field to the border, which is considered by the check part of the
entity (so no error message pops up) but for creation purposes, part of the
component.

As a result the field is positioned directly in the component as we can see in
the component structure, and over the border in the GFP. This is incorrect and
will lead to more unexpected behavior, like IDF hanging on the form compilation.

How to reproduce:

• Create a form with two entities.
• Give a border to one of them.
• Paint a field in the other.
• Move the field from one entity to the border of the other entity.
• Check the data structure and verify that the field directly under the
  component.
• (optionally) Compile. Uniface will hang.