towers should snap into slots

[davidak]

the game looks messy and unprofessional when the towers are not centered in the circle.

it should not be possible to place more than one object in one slot, it don't makes sense for the gameplay. also towers need to be removable (#37) since it don't makes any sense to have more towers than slots. (i will propose upgrades instead of combining towers)

towers should not be placeable freely on the map. mines should, as it makes sense.