Add field for operator:wikidata

[kjonosm]

See #168

[tyrasd]

This PR only defines the field, but does not add it to any preset. Without adding it to any preset, the field would be essentially useless.

Did you intend to add this field to any preset which includes the operator field? That would have the major downside of being a lot of effort to change, as it involves changing hundreds of presets. I think we should think about a better solution, e.g. involving an improved "wikidata" field which can directly work on a tag-triplet, e.g. here operator + operator:wikidata + operator:wikipedia. That would require some changes to the schema, however.

[kjonosm]

Did you intend to add this field to any preset which includes the operator field?

@tyrasd: That was the idea following the already featured subject:wikidata field. However, not to all presets with operator=* info but only those where operator:wikidata=* is commonly used, see NSI for reference. If you don't consider merging this pull request (and others adding *:wikidata fields) please let me know and I am happy to delete them.

[matkoniecz]

Without adding it to any preset, the field would be essentially useless.

What about objects where this tag was already added?

[kjonosm]

What about objects where this tag was already added?

The field still needs to be added to the respective presets...