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.
This didn't compile when I merged it and BevapDin's PRs together.

Full disclosure: your turret functions merge-conflicted and I may have not resolved them properly, but the functions that I attempted to resolve were both after this line.