Query Namespace

[askvortsov1]

**Part of flarum/issue-archive#286 **

Changes proposed in this pull request:
Move search results, search criteria, and ApplySearchParametersTrait to Query namespace.

They still extend (empty) search equivalents as a BC layer for extensions that expect those types.

Reviewers should focus on:
A few changes I'm considering, but am unsure about:

• Moving FilterState to query. It's shared, but it's also an implementation detail.
• Adding a QueryableInterface with a query method, and having both searchers and filterers implement it instead of filter and search methods. Leaning against this though because it isn't really necessary, and I don't see the benefits.

An actual extender for search drivers will have to wait until next release (or later), although that shouldn't be too difficult. Not a priority right now though.

[SychO9]

A few changes I'm considering, but am unsure about:
Moving FilterState to query. It's shared, but it's also an implementation detail.

I agree, what about having it and SearchState extend an AbstractQueryState ? might be better organized, FilterState would end up an empty class, but I think that'd be fine.

Adding a QueryableInterface with a query method, and having both searchers and filterers implement it instead of filter and search methods. Leaning against this though because it isn't really necessary, and I don't see the benefits.

Yea, We'd be overdoing it imo.