-heal subcommand to the /effect reimplemented
-healing_func() from old API standardized, included in new API
-(the last point is due to it being substantially different from others)
-fixed a few bugs, potential crashes
-fixed incorrect withering effect progression
-standardized variable naming
-improved the /effect command, allowing to use effect levels
-fixed a bug in level-to-factor conversions
-renamed effect icons to follow the new convention