Remove component name from `BACKDROP` class in the `defaultClassNames` object

[chris-pearce]

Looking at this object:

// Classnames likely to be shared across modules
export const defaultClassNames = {
    IS_ACTIVE: 'is-active',
    IS_HIDDEN: 'is-hidden',
    IS_READY: 'is-ready',
    NO_BACKDROP: 'no-backdrop',
    BACKDROP: 'dialog-backdrop'
};

BACKDROP: 'dialog-backdrop' should be BACKDROP: '[namespace]-backdrop' i.e. remove dialog. For all non-state hooks we'll need to prefix with our namespace as mentioned here: #24 (comment).

And do we need NO_BACKDROP as I thought if showBackdrop is false we won't render the backdrop <div> into the DOM?