Main changes
Some enemy projectiles can now be shot down by player (missiles, mostly)
Added UI icons for better readability of current ship state
Updated weapon powerup visuals for better readability
Another rework of Shotgun (projectiles are now faster, but less powerful and live much less)
Changes in some art assets on levels for better visual readability
Implemented second Secret tile type (so, now 2 secrets on Level 1)
Assigned a boss-type explosion to Boss 5