Convert from using string to enum values for severity in ErrorItem.
Storing and handling severity as string was the easy way earlier
but it is not convenient or efficient way to handle severities.
This commit is the first step in converting severity handling to
use the enum values instead of strings.
Ensure that native separators are used in saved reports. Reports may
be parsed by other programs/scripts so it is important that paths
are properly formatted.
CSV files are handy (and commonly used) to export/import data to other programs. CSV reports allow easy import of cppcheck error data to other programs for further processing.