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.
These tests are very complex and thus hard to understand. Could this be fixed by an operator?