Introduce classes for settings

[flixos90]

Let's add classes surrounding settings to provide a better structure for dealing with them. It will also allow us to get rid of some globals if we are in a position to remove them (in terms of BC).

Here is what I have in mind:

• A WP_Settings class should be introduced that contains get(), update(), add() and delete() methods. This will mostly be copy-paste from the related functions. The functions themselves will become wrappers.
• A WP_Settings_Registry will be introduced. It should contain all methods that handle registered settings (mostly introduced in 4.7). Again, the functions would become wrappers. We could get rid of the $wp_registered_settings global here and store these in a class property instead.
• The WP_Settings_Registry instance will be contained by the WP_Settings instance as a public property.
• A function wp_settings() will be introduced to access the WP_Settings instance or generate it if it does not exist yet. I'm not sure yet how to store the instance: The easy way is a global, but I was wondering where we're at with plans like a WP::get( 'settings' ) so that we could do it differently. Anyway, let's assume a global first.

I think it would be a good pattern to build the class in a flexible way, so that the registry instance and database instance are passed to the class constructor. The following is how I would envision the wp_settings() function:

function wp_settings() {
    global $wp_settings;

    if ( ! isset( $wp_settings ) ) {
        $wp_settings = new WP_Settings( new WP_Settings_Registry(), $GLOBALS['wpdb'] );
    }

    return $wp_settings;
}

I think once we agree on an approach, we should do something similar for metadata. But let's have the discussion in here first and open the other ticket afterwards.