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
* @param propName
	 *            the name of the property to get
	 * @return the property as a BigDecimal, or null if the value is not set or
	 *         cannot convert to a number
	 */
	public BigDecimal getNumberProperty( Module module, String propName )
	{
		Object value = getProperty( module, propName );
		if ( value == null )
			return null;
		if ( value instanceof BigDecimal )
			return (BigDecimal) value;
		ElementPropertyDefn prop = getPropertyDefn( propName );
		PropertyType type = MetaDataDictionary.getInstance( ).getPropertyType(
				prop.getTypeCode( ) );
		return type.toNumber( module, value );
	}
	/**
	 * Applies a design visitor. The derived element calls the corresponding
	 * visitMumble( ) method in the visitor. The visitor allows an algorithm to
	 * be implemented in a single visitor class, rather than all across the
	 * element inheritance hierarchy.
	 * 
	 * @param visitor
	 *            The visitor to apply.
	 */
	public abstract void apply( ElementVisitor visitor );
	/**
	 * Gets a property converted to a string value.
	 *