Allow multiple objects in cell

[ZivDero]

This PR converts Waypoint, Aircraft, Vehicle and Structure into Lists, allowing to place multiple of those entities in a single cell, which is supported by the game engine.

[Rampastring]

The game does not fully support this functionality. In particular, if buildings placed on top of each other, the game crashes if one of them is destroyed. This was a common mapping error with FinalSun/FinalAlert, which WAE was designed to prevent.

There are still a couple of good reasons for in turning a cell's buildings into a list though:

1. it has use for buildings that are converted to overlay. For example, placing a crate building with ToOverlay=CRATE under a real destroyable building, to add a collectible crate that the player can pick up after destroying the building
2. supporting maps that already have overlapping buildings

However I don't think WAE should allow placing overlapping buildings by default. Instead this should be behind some modifier key, so the mapper can't accidentally make buildings overlap, but instead it's always a deliberate action.

For units, aircraft and waypoints there is no such issue AFAIK, but is it beneficial to allow them to overlap? I have a hard time imagining a case where making units overlap would be very useful. It could still be allowed with the modifier key, but I'd also prevent it by default.

For the implementation, I think the current implementation might crash when deleting objects as you're modifying a collection that is being foreach'd. Deletion should instead act on a copy of the list.

[ZivDero]

YR does in fact fully support this and does not crash (whether the buildings are invincible or not). Although it seems that invincile buildings also get damaged when the vulnerable ones receive damage. I suppose adding a modifyer key wouldn't hurt, though.

As for deletion, in my tests it did not crash but I agree that this is not a safe way to do it indeed so will redo.
EDIT: In fact, it should be safe as I'm not performing actions on the collecttion that's being foreached, rather on a different collection.

[ZivDero]

Okay, all done. I've added Alt as the default hotkey that allows to place overlapping objects. Works for placing new buildings, moving/cloning buildings, as well as the Terrain copy tool. Don't think I've missed anything.

[Rampastring]

Functionality looks good now.

There are some stylistical things to fix, including some harmful renames.

[ZivDero]

Implemented the requested changes.