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.

* Made ammo qty reduce as flamethrower is used
* Blocked player from firing with insufficient ammo
* Set ammo to max when flamethrower is picked up
* Reset ammo at end of level
* Added status bar to show ammo amount
* Added event for ammo pickup