Bug 151959 inappropriate screen operation
steps to reproduce: - create a report - add a grid item (3x3) - switch to Outline view - expand the grid item to see its rows and cells - select the grid item, and drag it into any of its own cell expected behavior: - is it an invalid screen operation? if it is, it should be banned actual behavior: - the grid item was disappeared surprisingly
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
* @return The property value, or null if no value is set.
	 */
	public Object getProperty( Module module, ElementPropertyDefn prop )
	{
		Object value = getStrategy( ).getPropertyExceptRomDefault( module,
				this, prop );
		if ( value != null )
			return value;
		return prop.getDefault( );
	}
	/**
	 * Gets the search strategy for this element.
	 * 
	 * @return the search strategy for this element.
	 */
	public PropertySearchStrategy getStrategy( )
	{
		return PropertySearchStrategy.getInstance( );
	}
	/**
	 * Gets selector of the given slot of this element. The selector is kind of
	 * predefined style, and its style property value can be applied on contents
	 * of the given slot of this element depending on whether property can be
	 * inherited.
	 * 
	 * @param slotID
	 *            slot id
	 * @return the selector of the given slot of this element.
	 */
	public String getSelector( int slotID )