translate TranskribusMetadata into v>=2018 MetadataItem

[bertsky]

The valuable information does not have to be removed. Transforming not just the attributes, but also its recursive Property elements into MetadataItem/Labels/Label is worthwhile IMO.

[bertsky]

Also: at most segment types, we should convert Tag, Property and Link to something appropriate instead of removing them.

IIRC Transkribus uses these to label lines as "illegible" or "abbreviated" etc. Perhaps we should first make sure we understand the semantics and schema of allowed values before we map to Labels in PRImA.

[bertsky]

It would be really helpful to have an example page from Transkribus which heavily uses these features.
(Generally, a regression test would be nice to have...)

[bertsky]

Perhaps we should also synchronise with DTABf concordance...

[bertsky]

In particular: setting a correct @type for each predefined @custom, e.g. paragraph for poem_lg or other for closer.