Place tower with cannot-place-region

WeaponDetector will turn on-off according to whether it can place or not

Class modified:
	Enemy: add placing[bool] for HoverTest to manipulate WeaponDetector.
	Weapon: ShowDetector protected -> public
	HoverTest: add function to caculate minimum distance between point and
		line segment

Signed-off-by: Daeno <thomas820605@gmail.com>