Add a page locker while editing an existing pub

| Q              | A
| ------------- | ---
| Bug fix?    | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | none
| License       | MIT

Block the edit function using Zikula Core PageLock, if somebody is already editing the same page.
I have tested this code using Clip 0.9.4 for zk1.3.x
Philippe