The board setup dialog controls DRC error visibility, so the
two dialogs shouldn't be open at the same time to prevent
data contention.
Follow-up to https://gitlab.com/kicad/code/kicad/-/issues/4023
Replace SEVERITY_ERROR by RPT_SEVERITY_ERROR to avoid collision with a system definition.
Replace other SEVERITY_XXX by RPT_SEVERITY_XX for consistency.
Fix compil warnings and some other compil issues.