Create a base WP_Backstage_Field class, and move out all field code to classes that extend this class

[theenoahmason]

This should have all of the methods needed to construct a new field in a more normalized way, and will allow us to get methods out of WP_Backstage and WP_Backstage_Component classes into a more appropriate place. This will also pave the way for extensions and extra fields to be added in a normalized way.

See: https://github.com/dreamsicle-io/wp-backstage/wiki/Development#developing-fields

The interface should have a signature similar to this:

class WP_Backstage_field {

    public $type = 'text';
    public $default_args = array();
    public $is_non_regular_text = false;
    public $is_remove_label_for = false;
    public $is_textarea_control = false;

    public function __construct() {}

    public function init() {}

    public function render() {}

    public function sanitize() {}

    public function format_value() {}

    public function format_column_value() {}

    public function inline_style() {}

    public function inline_script() {}

}