Common - Add keybind and function to unload unit's weapon/muzzle

[Drofseh]

When merged this pull request will:

• add keybind and function to unload weapon/muzzle of unit
• requires Common - improve CBA_fnc_addMagazine and CBA_fnc_addMagazineCargo CBATeam/CBA_A3#1527

To Do:

• document new weapon config entries, ACE_unloadAction, ACE_unloadSound. I'm not sure which framework doc to put this in.
• see if there's a way to interupt the reload animation if there is no ACE_unloadAction

[Drofseh]

I haven't been able to find a way to interupt the default reload animation, playing another gesture (or no gesture) doesn't seem to do it.
However, even when the full reload animation is played, because the magazine disappears in the middle it looks like they are just clearing the chamber, so I'm happy with it as is.

[Drofseh]

This is ready for review now that CBATeam/CBA_A3#1527 has been merged

[PabstMirror]

I can mash the button and multiple sound effects play
but it doesn't dupe ammo so it's probably not a problem

[PabstMirror]

also maybe this should be in ace_reload?

[jonpas]

I think I recommended common as I saw potential uses for it in other modules.