Back-port 8949 to 7.9 - Prevent same-name siblings for folders/documents (merge and improve from branch)

CMS7-8203 has worked on preventing same-name siblings for folders on a forked of the branch 7.8. This work needs to be merged to trunk and fixed following test-cases:

For following scenarios the fix fails:
1 - When user attempts to change the URL to non-existing one, then it is possible to create a folder or document with same name
2 - During translation of a document, if existing document name or folder name is found the error message shown is “The action failed” generic and not user friendly
3 - At root level it is still possible to create folders with same name (happens at gallery also)