Fixes, fixes and fixes!

Lots of fixes for a better experience:

• Added a sound that alerts you when an enemy is getting too close (useful in messy situations when visibility isn't enough)

• Improved damage for the Machine gun auto turret gadget (and slightly increase the amount of ammo)

• Improved damage for the Machine gun turret in the "garden"

• Now you can see wall tunnels from the map (once you got close enough, they appear in the map with a different color/pattern)

• Now you can see Upgrade Stations from the map (once you got close enough, an icon will appear in the map)

• solved a problem in the inventory caused by dropping-picking up-dropping again a weapon (could be related to inventory freezes that occasionally occured)

• now dropped boxes closed to each other merge together for visual clarity

• dropped boxes are now visible in the map (little square icon)

• empty dropped boxes are deleted after looting