Bug 122850 Table/list should not be able to be inserted in Master Page mode.
Description: Table/list should not be able to be inserted in Master Page mode. Steps to reproduce: 1. New a report and transfer to Master Page mode. 2. New a datasource and dataset 3. Drag a list/table to page header/footer, bind it to dataset and drag some columns inside. 4. Save and preview Expected result: Insert list/table item in master page should be forbidden. Actual result: They are allowed and preview showed "undefine" in page
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
assert child != null;
		assert child.getExtendsElement( ) == this;
		assert derived.contains( child );
		derived.remove( child );
	}
	/**
	 * Sets the parent element. This version should be used to implement a
	 * command to change the parent element. The caller must have validated that
	 * the new parent is valid for this element.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @param base
	 *            The new parent element, or null if the element is to have no
	 *            base element.
	 */
	public void setExtendsElement( DesignElement base )
	{
		DesignElement oldExtends = getExtendsElement( );
		if ( base == oldExtends )
			return;
		if ( oldExtends != null )
			oldExtends.dropDerived( this );
		if ( base == null )
		{
			extendsRef = null;
		}
		else
		{