ReorderContentWidget to sort content via drag and drop / little adaption to ContentContainerController

Reorder by drag and drop:
- ReorderContentWidget is a configurable javascript widget that uses jquery-ui sortable and ajax requests to send a reorder request to the server
- ReorderContentBehavior contains the logic to process such a request and reorder the elements in the database. The method used for that is also configurable for specific needs.
Adaption to ContentContainerController:
- small change. The controller now also accepts sguid/uguid as param in a POST request.

EDIT -> don't know if the last commit is also in this pull request... seems so, so I add a comment to the last commit here:
- The ReorderContentWidget was attached to the admin view in the last commit. I wanted to check that this does not need a lot of code changes to existing modules.

Luke/Struppi, please have a look if the safety and validation of the added code is allright (Controller Actions). I am not sure here.