API Cleanup inconsistent SiteTree::duplicate API

[tractorcow]

Merge this before accepting symbiote/silverstripe-advancedworkflow#261

At the moment onBeforeDuplicate is run on both the original and the duplicate.

This corrects the behaviour to run only on the duplicate, and have it pass in the original to the first parameter consistently.

dataobject implements as:

public function duplicate($doWrite = true) {
        $className = $this->class;
        $clone = new $className( $this->toMap(), false, $this->model );
        $clone->ID = 0;

        $clone->invokeWithExtensions('onBeforeDuplicate', $this, $doWrite);
        if($doWrite) {
            $clone->write();
            $this->duplicateManyManyRelations($this, $clone);
        }
        $clone->invokeWithExtensions('onAfterDuplicate', $this, $doWrite);

        return $clone;
    }

[tractorcow]

Fixes #1253