Visit ObjectValue nodes when validating unused variables

[nigel-sampson]

Resolves an in issue where a variable used in a decomposed complex input type was not being validated as being used.

query queryWithComplexInput($name: String) {
    findDog(complex: { name: $name }) {
        name
    }
}

Fixes #745
Fixes #215

[michaelstaib]

Can you also have a look at variables in lists?

[michaelstaib]

I have updated the changlog and will merge this one as soon as the build becomes green.