CanFire boolean property for HDFirearms

[pawnishoovy]

Is your feature request related to a problem? Please describe.
There is no surefire way to know whether a HDFirearm is currently able to fire. Educated guesses can get you halfway there (is there a magazine, are we reloading) but there's no way to account for RPM limiting for instance.

Describe the solution you'd like
HDFirearm.CanFire (or similarly named) properly that is true when the gun is able to fire that frame, and false when not. This would reflect whether you could press "attack" right now and have the gun go bang/start to go bang.

[garethyr]

Note: doing this would probably require rewriting most of HDFirearm's logic to integrate it in so the HDFirearm sets the CanFire flag and then uses its value when firing.
Do this when HDFirearm is cleaned up.

[MaximDude]

Resolved in #480.