Update v0.4.0.0

Achievement get: add achievements

As I planned before, I added a global progress: achievements, unlockable difficulties and challenges. Hard difficulty used to be unlocked in previous versions, but I want to message a player that Normal difficulty is supposed to be hard. Old Easy difficulty is removed, so Medium is Easy now.

In demo I added 5 achievements and 1 challenge. Some achievements are obtainable on Normal or above, like most of them supposed to in release build. The "Bleed" challenge is unlockable after beating first part of the demo, when you defeat Angel.

This also required an improvements of save file and inventory code. I can add new entries in save file without necessity to update its version. As for inventory: before this update number of items in active slots was hardcoded to nine, but most likely I won't add anything new on release.

Expect bugs.

Changelog:

- Updated save files
- Updated difficulties
- Added achievements, five in demo
- Added challenges, one in demo
- Updated inventory code
- Several map changes
- Decreased health of some enemies and Angel
- Increased Anti-materiel rifle damage
- Same sound can't be played in one tick
- BGM storage can be changed in config file
- Fixed bug when text doesn't move in message box
- Fixed bug when light rendered after bash with every weapon