Proper reversal of relations in sub queries

[nilmerg]

Changes the way relations can be reversed drastically as it is now possible to influence the relation to use during reversal with ::setReverseName(string) which allows Icinga DB Web to drop the error-prone to.from and from.to relations.

An additional change is that it is now not mandatory anymore to define relations that are solely being required because of sub-queries. Missing relations on the reversed path are automatically registered. For this, each relation type now has its specific counterpart which is possible to override with ::setReverseClass(class-string). The default however, is to use the same type which is the case for BelongsToOne and BelongsToMany. For BelongsTo a sane override has been chosen that is based on how it's used at the moment in our products, as HasOne and HasMany may both be appropriate. But the latter clearly is used more often.

Since ::reverse() uses the source's table alias by default as reverse name, a deprecation notice is triggered if the original forward path uses a different name, indicating that it is necessary to use this name as explicit reverse name.

fixes #170

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A substantial preparation for this is the introduction of Relation::bindTo(Model, string, Resolver) in order to pass control to relations how they're prepared. Since the introduction of BelongsToMany, it is established that a relation may resolve to multiple hops and thus needs to perform steps n-times rather than a single time. It's this reason because registering the alias and resolving the filter is now a responsibility of a relation rather than the resolver.

Relations know it better how to and the override of bindTo in BelongsToMany proves it as it turned out that it is necessary to allow referencing the junction table in either the filter or the through filter in order to be able to better reverse relations.

Qualification must be done by a relation in turn as well, as otherwise there's a mis-match with what's allowed to reference and what can be qualified. My initial attempt was to teach Relation::resolve() this, but without passing it the resolver and changing the return value this doesn't make sense. Sadly, this is out of the question as this is a breaking change. Say hello to Relation::setFilterSubjects() due to this.

[nilmerg]

@BastianLedererIcinga Please note that this must not break anything in other products. Icinga DB Web for example must still work without Icinga/icingadb-web#1398, albeit with a deprecation notice here and there.