Bug 169025 Supports overridden on editor contribution
After contributting a new editor to the multiPageReportEditor, supports to enable the new editor to override a editor by assigning the ID.
UI/org.eclipse.birt.report.designer.ui.editors/src/org/eclipse/birt/report/designer/internal/ui/extension/EditorContributorManager.java
/**
	 * Gets the extended element point with the specified extension name.
	 * 
	 * @param extensionName
	 *            the extension name of the extended element
	 * 
	 * @return Returns the extended element point, or null if any problem exists
	 */
	public FormPageDef getFormPageDef( String targetEditorId, int index )
	{
		Assert.isLegal( targetEditorId != null );
		Assert.isLegal( index >= 0 );
		EditorContributor editorContributor = getEditorContributor( targetEditorId );
		if ( editorContributor != null
				&& editorContributor.formPageList != null )
		{
			return (FormPageDef) editorContributor.formPageList.get( index );
		}
		return null;
	}
	private void createEditorContributorMap( )
	{
		synchronized ( this )
		{
			editorContributorMap = new HashMap( );
			for ( Iterator iter = getExtensionElements( EXTENSION_MULTIPAGE_EDITOR_CONTRIBUTOR ).iterator( ); iter.hasNext( ); )
			{
				IExtension extension = (IExtension) iter.next( );
				IConfigurationElement[] elements = extension.getConfigurationElements( );
				for ( int i = 0; i < elements.length; i++ )