Bug 193245 Should block user to set content type property on the child element
After we switch from the slot to content element type property, all the rules that we applied to the slot before should be applied to the content element type property. When an element is extended from the library element, it should not be allowed to change the child element structure. So, when user call setProperty() to set the value of the content element type property, Model should throw out exception.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
*            the new property value.
	 * @throws SemanticException
	 *             if the property is not found.
	 */
	public void setProperty( String propName, Object value )
			throws SemanticException
	{
		checkAllowedOperation( );
		propName = StringUtil.trimString( propName );
		
		ElementPropertyDefn prop = element.getPropertyDefn( propName );
		if ( prop == null )
			throw new PropertyNameException( element, propName );
		setProperty( prop, value );
	}
	/**
	 * Sets the value of a property. If the mask of a property is "lock", throws
	 * one exception.
	 * <p>
	 * If the mask of this property has been set to "lock", no value will be
	 * set. To set the value of a property, the mask value must be "hide" or
	 * "change".
	 * 
	 * @param prop
	 *            definition of the property to set
	 * @param value
	 *            the new property value.
	 * @throws SemanticException
	 *             if the value is invalid or the property mask is "lock".
	 */