Added Heavy Fighter Cockpit. Costs more steel plate and metal grids in hopes of making it stronger

Beeg update. 2 new cockpits- SB SLTH and the Heavy Inline Cockpit. Heavy Inline is meant to be able to take a bit more damage with different build costs but bad visibility. Requested by Ivan of Spades. Requested by Farrell: Centerline Rover Cockpit with no back, 1 block wide to fit into the Rover Cockpit halves. Added Backless variants of the Left and Right Rover cockpits