Add API support to schedule.module

I needed to add some additional functionality to scheduler. I could have hacked up scheduler.module but that's not the way to do it. So, the simpler option was to add in a small amount of code to scheduler.module to allow it to call an external hook when it's posting or hiding a node.

Other modules can then extend the functionality, so that's what I did. I'd like to get the small change to scheduler.module reviewed and if approipate commited.

The alterations are in the patch.

Note, the patch supplied above was created by first patching scheduler.module with scheduler.module_2.patch from #52545. This newly created file was then patched again by scheduler.modile_moderate.patch.txt from #61495

Thanks and regards
--AjK