Rewrite of `CInifile`

Created by: AMS21

• Completely handwritten parser which parses the entire ini file in one go including conversions to lowercase and without any string copies
• Fixed all sorts of crashes, infinite loops etc. I could find for invalid ini files. Instead of crashing we now report an (hopefully) helpful error message
• More lenient parsing for #include or inheritance thus

[a]
  #include "other_file"

[b] : a

will now be parsed just fine.

• Renamed Items members first and second to name and value
• Added and improved const-correctness for the entire CInifile class
• Moved all function definitions to the source files
• General cleanup of xr_ini.h and xr_ini.cpp