Amend list of xAPI properties that are saved

[otacke]

Check what xAPI properties might make sense to keep in the database, e.g. the optional properties of interaction activities.

[glisapn90]

When solving Drag and Drop interactivity, H5PxAPIkatchu does not record data outside xAPI column (name of drop zones, name of images). But in xAPI (json) store the complete xAPI statement of Drag and Drop interactivity ( "source" or "target" fields in xAPI). I think it would mean that these two fields are visible outside the xAPI field.

[otacke]

@glisapn90 Sure - why would I save values to the database if I don't offer a way to display them?