Walls, Gates, and Wall Towers need to ignore other walls related obstruction zones

[Kieran P]

If you try to build a wall, it's very hard to make one without gaps, because the obstruction zone prevents overlapping walls to make a smooth wall.

So, walls, gates and wall towers need to ignore the obstruction zones of other walls, gates and wall towers. They should still be unbuildable if placed on things like rocks or trees, but for e.g., when building a wall, overlapping another wall and clicking build should be possible.

[Kieran P]

Will be handled in #609

[Kieran P]

Or rather, will be handled in #619