Add support for +attack3 for "buttonmode"

[shadow93]

So, i was talking with Friagram over on steam, and he has pointed out that one of the changes done in the recent TF2 patch breaks alt-fire for medic bosses equipped with a bonesaw melee.

Reason being, because pressing alt-fire with a bonesaw makes the user perform a taunt. Attempting to remove the condition via OnConditionAdded will cause the condition to reapply over and over again.

Although for medic bosses that don't gain a weapon on-rage can use RELOAD instead, this isn't really an option for medic bosses that gain a weapon on-rage.

I think it's time +attack3 (Middle Mouse) is added as a valid buttonmode button.

This would also fix scenarios with RELOAD abilities and ranged weapon bosses.

[Starblaster64]

Actually, it's only the Ubersaw and Amputator that are affected. The rest don't do anything on alt-fire. The alt-fire on the Amputator makes sense because it's on the attribute. The Ubersaw sort-of makes sense because the taunt-kill is mildly useful.