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.
@glye because as by default and admin user have access to everything, it looks a bit weird to me telling that he/she has no **access** to do that. 
Same for line 380, yes. There's no policy that denies an admin user to swap a node. If such policy existed it would deny to do the swap operation entirely, no matter if the destination node is a container or not... 

Just my 2 cents :).