Select timezone

Adds a standalone time zone select dropdown. The time zone select can be used with standalone `text_datetime_timestamp` if desired, which can be useful when multiple datetime fields are used in the same metabox, and they're all in the same timezone. Can be configured like so:

    array(
        'name' => 'Time zone',
        'desc' => 'Time zone'
        'id'   => $prefix . 'timezone',
        'type' => 'select_timezone',
    ),
    array(
        'name' => __('Start time', 'domain'),
        'desc' => __('Date and time for conference opening', 'domain'),
        'id'   => $prefix . 'start_timestamp',
        'type' => 'text_datetime_timestamp',
        'timezone_meta_key' => $prefix . 'timezone',
    ),
    array(
        'name' => __('End time', 'domain'),
        'desc' => __('Date and time for conference closing', 'domain'),
        'id'   => $prefix . 'end_timestamp',
        'type' => 'text_datetime_timestamp',
        'timezone_meta_key' => $prefix . 'timezone',
    ),