SPI: changed relation type to string

changes: every relation (src, dest, type) is stored now by SPI. This makes it easier to handle cases where multiple relations of the same type occur. The BL can now simply remove each relation separately . But it must aggregate the relations of the same type view viewing e.g. in GUIs.
SPI: renamed created,modified to creationDate, modificationDate
SPI: added visit data to user value (see also sql-ng schema)
SPI: added discriminator in content for content classes (i.e. content, user, asset etc)
SPI: create/update structs for section, adapted handler