[5.7] Allow adding additional $manyMethods when extending the model class

[spawnia]

Motivation

This allows adding additional $manyMethods when extending the model class
and still receive a correct guess for the relationship name.

I have to overwrite the BelongsToMany relationship class in my application.
To return such a relationship, i also extended the Model class with a custom
method that returns an instance of my particular relationship clas.

Changes

Access the $manyMethods property through static::$manyMethods instead of
Model::$manyMethods. This both reduces coupling between the trait and the Model
class and allows for extensibility.

Removed the name of guessBelongsToManyRelation from the list but still consider
it when analyzing the call stack. This makes $manyMethods more slim and easier
to override.

Removed findFirstMethodThatIsntRelation from $hasMany array, since it does not
appear anywhere in the code and seems to have been forgotten there.
It is also not really a name for a many-relationship method, so if it is needed,
i think there might be a better place for it.