Add OnBeforeItemValidate event

I propose to add OnBeforeItemValidate event to kDBEventHandler class. Event should be called, when someone calls kDBItem::Validate method. This can be used to perform custom validation instead of OnBeforeItemCreate/OnBeforeItemUpdate as for now.

I propose this, because I have project, when kDBItem::Validate method is called on it's own, not from kDBItem::Create/kDBItem::Update methods, and my custom validation checks in OnBeforeItemUpdate/OnBeforeItemCreate are not even called.