Don't Use Fixed Port Positions During Layout Unless Necessary

[philip-alldredge]

The layout code currently uses the FIXED_POS option as a port constraint. This is needed to properly layout feature groups which have children. However, in cases where such features groups are not present, the FREE or FIXED_SIDE port constraint could be used.

Suggest used FIXED_SIDE in that case and to add a preference that will allow the user of FREE. Experiments have shown that FREE produces better results than the current solution but could result in output features being on the left of the container.