Implemented a safe way to build a schema

[vepanimas]

Partially addresses these issues #348 #344 #340 #201 #313 #309.

The main goal of this PR is to avoid re-declaration errors during the schema build and not to fail completely in such cases:

type Query {
  user: User
}

type Query {
  cart: Cart
}

type Query {
  user: User;
}

extend type Query {
  user: User;
}

This situation occurs quite often when the schema is divided into several files and is collected during the build or in runtime. For example, this can often be found when using graphql-modules.

It doesn't fix error messages though, they are still pretty confusing and useless because they don't help you to find where the re-declaration is located. I think we should completely ignore them from graphql-java and implement custom inspections, which could be configured or even disabled by the user.

Also we should support resolving multiple declarations using PsiPolyVariantReference.

[anstarovoyt]

addDefinition(T)?