Add clear to list classes

[Benestar]

This is needed as a replacement for users of the holder interfaces to remove all elements from one of the given list classes.

Bug: T128363

[thiemowmde]

There is Fingerprint::setAliasGroups, so we should not need this.

[JeroenDeDauw]

Looks good. Could perhaps also test that an empty list does not change state. But not that important

[thiemowmde]

@Benestar and I had a look in our code base. Do we really need these methods, and where? What we found:

• EntityChangeFactory clears statements, descriptions and aliases for performance reasons. This is a hack, see T113468.
• Deserializers are currently written in a way they first create an entity and then set all labels, descriptions and aliases in one step. This can be rewritten: For example, first deserialize all labels, construct a Fingerprint from that, and then construct an Item from that. No clear nor "setTermList" or something like that needed, only constructors.
• There are "patchers" that manipulate terms in a very, very complicated way. This can be refactored. A label patcher, for example, should call removeLabel, setLabel, and nothing else. It should never create a new TermList and replace the old one. It's not a "patcher" if it does this.

[Benestar]

On hold, should not block 5.1.0

[thiemowmde]

Rebased. Questions:

• Do you think we still need this in 6.0?
• Or is this the opposite of Remove clear from Entity, Item and Property #614 and should be closed when we merge the other one?

[thiemowmde]

What I wrote above is still true. There is still code that needs to clean these list classes. Refactoring all this code in a way that it does not need to do this is a lot of effort, and probably not really worth it.

I strongly supported #614 because it should not be possible to "repurpose" an existing entity and turn it into something completely different. There really should not be an Item::clear, on no entity type. But this here is very different. These are list classes. It's not really "repurposing" if code cleans a list of aliases.

Considering the odd fact that I just implemented a clean in wmde/WikibaseDataModelServices#157 I suggest to merge this patch now. Pinging @brightbyte for the sake of completeness.

[manicki]

this should have been 7.0 (or maybe 6.4) I guess

[thiemowmde]

Thanks! Fixed in #719.