Add buildable structure: Door
Player and partner can pass through doors. The pathfinding system of partner' characters will treat the door's position as passable, and enemy characters will attack the door.
Other:
Doors will have an opening animation when the player or partner character is near the door, and a closing animation when the player or partner character is far away from the door
Changed building flags to be passable areas, previously player would be blocked when approaching the flag, now they can pass right through the flag
Although some buildings are passable, it doesn't affect the fact that only one building can be built in a construction grid.