Implement `WordPoints_EntityishI` in `WordPoints_Entity_Array`

[JDGrimes]

The WordPoints entity API consists mostly of classes which descend from WordPoints_Entityish and thus implement WordPoints_EntityishI. One exception is the WordPoints_Entity_Array class. This fact means that some methods accept WordPoints_EntityishI|WordPoints_Entity_Array, which is quite a mouthful. It would be cleaner if the entity array class was just an entityish, instead of being an odd man out.

In order to make that happen, the entity array class will need to implement three methods:

• get_the_value() — Actually, this could be achieved by extending WordPoints_Entityish instead of implementing the interface directly.
• get_slug() — It would make sense to implement this and have it return "$entity_slug{}". This would also give us parity with the JS behind the entities in the UI.
• get_title() — This is the hardest one to justify. It doesn't exist because we just don't need it. And because of i18n, it is awkward.

The main issue is get_title().

[JDGrimes]

These are our options with get_title():

• Return a basic string like "Array".
• Incorporate the title of the entity into the title, like "%s array".
• Just return the entity title.
• Just return an empty string.

The first two have the issue that we'd probably want to make this localizable, though since we don't use it we really wouldn't have to. But we'd need to figure out how to handle the fact that most users would probably have no idea what an "array" was.

The last option is really lame, even though we don't use this feature right now. But if we were going to just return nothing, we haven't really implemented the interface, have we? We could actually remove get_title() from the entityish interface, but that would probably make things worse instead of better. So I think we should pursue actually returning something meaningful from this.

So, if we need to decide how to describe this thing. What word can we use instead of "array"?

• set
• collection
• group

"Set" seems the best fitted in my judgement.

We musts also keep in mind that we have only the singular name of the entity, so we can't say "Set of %s", we'd have to say "%s set" instead, which seems strange. "%s collection" seems better.

[JDGrimes]

Let's just go with %s collection for now.

[JDGrimes]

This fact means that some methods accept WordPoints_EntityishI|WordPoints_Entity_Array, which is quite a mouthful.

Interestingly, most of these methods were documented as accepting WordPoints_EntityishI specifically, even though some of them could actually get passed entity arrays.