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
* 
	 * @return The element definition. Will always be non-null in a valid build.
	 */
	public IElementDefn getDefn( )
	{
		if ( cachedDefn == null )
		{
			cachedDefn = MetaDataDictionary.getInstance( ).getElement(
					getElementName( ) );
			assert cachedDefn != null;
		}
		return cachedDefn;
	}
	/**
	 * Adds a user-defined property. The property must be unique within the
	 * inheritance hierarchy of this element. It cannot duplicate either another
	 * user-defined property or a system-defined property. These checks must
	 * have been enforced by the application prior to calling this method.
	 * <p>
	 * Part of: User-defined property system.
	 * 
	 * @param userProp
	 *            definition of the user-defined property
	 */
	public void addUserPropertyDefn( UserPropertyDefn userProp )
	{
		assert getDefn( ).allowsUserProperties( );
		assert userProp != null;
		String propName = userProp.getName( );
		assert getUserPropertyDefn( propName ) == null;