Per-turret disable/enable/burst mode

Similar to individual engine selection, except for turrets.

Each turret can be enabled, disabled or in burst mode. Burst mode isn't offered for turrets with no burst capability. When selecting turrets, screen will center on highlighted turret's location with an 'X' denoting its position.

Adds a new int to amount/direction/open union (turrets didn't use this union before, right?). Currently uses only values 0, 1 and 2 for disabled, enabled and burst.

By default all newly installed turrets are disabled.
My only objection is that this isn't available when firing the weapon on foot, so folks are gonna clamor for that ability. It may be worth making non-Blaze turrets (which already have this sort of thing, or at least the high-end ones do) need additional electronics to justify computerized burst-limiting/charge-control.

(Moved from your repo to the diff, more accessible, sorry)