Added methods for creating EntityBullets

Kind of duplicate methods for creating bullet entities, but can be
overriden to allow sub classes of EntityBullet to be used
Migrated plane to new spawn method

Plane used ItemBullet.getEntity doesn't create a bullet on its own
Migrated mech to new spawn method
Migrated vehicles to new spawn method
Migrated seat weapons to new spawn method
Migrated MG and AA to new spawn method
Migrated hand held guns to new spawn method