Allow Preassemble flamethrower inside bagpack

[Mouse]

you can use already use a screwdriver on flamethrower frame, making it only require three slots (1 air tank, 1 fuel tank, 1 for the assembly)

IIRC the unsecured state for the welder/rods/igniter assembly was removed, meaning that only the secured state (which is the flamethrower) still exists.  Unless behavior was changed (I haven't tested it), that state doesn't fit in a backpack.