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.
Well you can *technically* do so on foot, you just have to fire it as soon as it reaches the level of charge you want to use. I'm guessing the turrets would work the same way, simply firing the moment they reach the desired level?