Improve translation handling

Currently, strings are translated as follows:

```
esc_html( apply_filters( 'hidden_posts_checkbox_text', 'Hide Post' ) )
```

This implementation forces users to translate strings via filter. While this works for tech-savvy users, it does not work for the average user. Instead of handling the translations via filter, I'd create a *.po file, so that the plugin strings can easily be translated via the WordPress translation interface, similar to https://translate.wordpress.org/projects/wp-plugins/jetpack/.