Add flamethrower ammo back

I wanted to add this back. Having finite ammo adds more depth to the
gameplay. I set the max ammo high so that the pace of the game can keep
up and not be constantly broken by the need to collect ammo; it's just
a matter of balance.

* Added status bar for ammo amount
* Added event for ammo pickup
* Set ammo to max when flamethrower is picked up
* Blocked player from firing with insufficient ammo