[NodeBundle] Dragging pageparts to other section

[dannyvw]

There is a problem that can occur when editing a page with multiple sections. The issue is that it is possible to drag pageparts into other sections. For example, from the "main" segment to the "header" segment. You will see a red block, but it doesn't do anything. You can simply save the page. After saving, the pageparts' names are broken. If you drag them back to the correct section, they will regain the correct names (after saving the page).

The best way to fix this problem is to not allow pageparts to be dragged to other sections. If that is difficult to implement, the secondary solution is to add validation so that it's not possible to save the page when the pageparts are not in the correct sections.