Support for OSM wikibase key type (P9) property as a source

[cuatim]

Hi,

Usage of https://wiki.openstreetmap.org/wiki/Key_types in OSM wikibase is growing. And since key types can have formatter URLs set, this can be a good fallback/addition for the formatter URL of the main item.

An example with key type opening hours:

• Key:opening_hours:atm or Key:collection_times do not have a formatter URL set, but their key type opening hours has. So the formatter URL from opening hours could be used.
• Key:opening_hours has its own formatter URL set, so keep as as.

Best,
C