improved firing controls from double barrelled tanks

[Will0hlep]

after playing the DBTs for a while I've decided this might be an idea worth sharing

1) designate a key or other button ('x' or 'center click') to change between single fire and double fire modes
2) when in single fire mode there would be 2 advantages:
                     I) you fire as you press the left mouse button (or after the duration of the minimum mouse press that the WG server will accept), not as you release,

                    II) holding the left mouse button results shells being fired as soon as they were loaded (like in a normal tank)

3) when in single fire mode it would be cool to have 2 options for how to fire (selected by options menu or config):

                     I) holding left mouse down for salvo time, release mouse to cancel,

                    II) press left mouse to start salvo time, press left mouse to cancel