Main constructor
Parameters: position - the Position instance needed by the addAreas stage of the LMs. Parameters: side - the side to which this space element applies. Parameters: condLength - the length-conditional property for a border or padding specification Parameters: isFirst - true if this is a padding- or border-before of the first area generated. Parameters: isLast - true if this is a padding- or border-after of the last area generated. Parameters: context - the property evaluation context