Support for update_metadata_by_mid

[soft79]

This pull request adds support for the update_{$meta_type}_metadata_by_mid-filter.

In WordPress the function update_metadata_by_mid can be used to update a meta value by using the meta-id. This function doesn't call the filter update_{$meta_type}_metadata which WP-Multilang uses to update translations. WP 5.0 introduced the filter update_{$meta_type}_metadata_by_mid which can be used by WP-Multilang instead.

For example: update_metadata_by_mid is used by WooCommerce when updating custom fields of products. When using the following wpm-config.json the field test_field_for_wpm would not be translatable without this pull request.:

{
  "post_fields": {
    "test_field_for_wpm" : {}
  }
}