Portage: Transport Ships on Land

Adds a Naval Porter unit, unlockable on Navigation II in Antiquity and available always in Exploration and Modern. It acts as a 'canal'-lite unit, you can naval units into it and unpack on the other side. You can technically move however much you like with the naval units packed, but I have reduced the units movement to one when packed. Also, the naval units will be lost if the porter unit dies while on land and not near naval tiles. The Porter unit has the Initiative effect much like a commander, so should be able to deploy the unit without losing its movement. The unit does not gain experience, it is retained on era transition, but will not contribute to keeping extra units on era transition. Currently I cannot find a way to get the upclose view of the unit in the panel to not be a blackscreen, and the ships visually look like they are moving through water, but these are minor visual only issues.