Fix EZP-23170: swapping node with sub items with non-container node.

JIRA: https://jira.ez.no/browse/EZP-23170

Problem: swapping a non-container node (class definition) with a node that contains children/sub items will result in an invalid state;
The new node will display the sub item count but will not list any children on the admin interface.

This PR adds an additional check in content/action that prevents the swap if any of the nodes is not a container and the other contains children.

Tests: manual.

Note: 
An improvement to this would be to add the check in the interface, before the action is executed (for example when browsing for the "destination" node.
@lolautruche The first line of the test, which was there before, I don't understand either. (Why can't you move non-container nodes?) The rest of it, on multiple lines, seems clear enough (FWIW, I wrote it). An operator would just move the complexity elsewhere imho, though it might make things easier for someone reimplementing this template.