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
* inherited.
	 * 
	 * @param slotID
	 *            slot id
	 * @return the selector of the given slot of this element.
	 */
	public String getSelector( int slotID )
	{
		ElementDefn defn = (ElementDefn) getDefn( );
		SlotDefn slotDefn = (SlotDefn) defn.getSlot( slotID );
		if ( slotDefn == null )
		{
			return null;
		}
		String slotSelector = slotDefn.getSelector( );
		if ( StringUtil.isBlank( slotSelector ) )
		{
			return null;
		}
		return slotSelector;
	}
	/**
	 * Returns property value with predefined style.
	 * 
	 * @param module
	 *            module
	 * @param prop
	 *            definition of property to get
	 * @param selector
	 *            predefined style