Implement warnings in checker

- Turn some non-critical errors into warnings
- Verbosely mark errors/warnings as such
- Implement error/warning class numbers for easier visual differentiation
  between error/warning groups
- Turn missing icons into warnings as well
- Check for unused icons, yet again counted as warnings
Remove misspelled icon, found by updated checker
Add moretags checks
Merge pull request #29 from AMDmi3/check-warnings

Improved checker, implemented warnings