Implement wall gates

[vts]

The current wall system as implemented in #786/r11760 does not yet have a way to add gates to an existing wall. It should be possible to select a single long wall segment and click a UI button to transform it into a gate, at a certain cost.

From an implementation point of view, this is likely to require support for multiple obstruction shapes per entity. Specifically, for the gates to operate properly, we'll need a way for a part of the obstruction box corresponding to the physical gate part in the model to be an independent shape, on which we can toggle the BLOCK_MOVEMENT flag to allow units through while open, or block them while closed.

[!!Chaos!!]

I think a transform function alone could handle it in that it would allow us to first transform a wall into a closed gate and then transform the closed gate into an open gate as needed. The open gate could just operate like the farms do where allied and enemy units could just walk right through. This seems more sensible to me then AoE/M's way of having an open gate still block out enemies as if they were vampires or something. I suppose you could code it to block everyone when closed and allow everyone when opened, but I really want a reusable transform function to use for a bajillion other things.

[Badmadblacksad]

In 12081:

Gates. Adds UI buttons, replaces wall section with a gate, defines and create obstruction shapes, detects friendly units and eventually disable door's "block movement" flag, allows us to lock / unlock the door by disabling "block pathfinding" flag. Needs icons, play sound, animations. Fixes #1385, refs #619.