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.
XML report format 1 only had error "lines" that we promoted to "items"
when adding to GUI. XML report format 2 contains error "items" so
change the code to read items and do the promotion directly when
reading and parsing the data.
Kimmo Varis