Snippet: Use classes instead of inline styles for text alignment

Note This answer was originally included in @bitstarr‘s question above and was included as a separate answer here to comply with WPSE’s Q&A model.

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Maybe someone else will have this issue and so i will share my solution here with you folks.

function make_mce_awesome( $init ) {
    /*
        There are easier things than make 'left/center/right align text' to use classes instead of inline styles
     */

    // decode
    $formats = preg_replace( '/(\w+)\s{0,1}:/', '"\1":', str_replace(array("\r\n", "\r", "\n", "\t"), "", $init['formats'] ));
    $formats = json_decode( $formats, true );

    // set correct values
    $formats['alignleft'][0]['classes'] = 'text--left';
    $formats['aligncenter'][0]['classes'] = 'text--center';
    $formats['alignright'][0]['classes'] = 'text--right';

    // remove inline styles
    unset( $formats['alignleft'][0]['styles'] );
    unset( $formats['aligncenter'][0]['styles'] );
    unset( $formats['alignright'][0]['styles'] );

    // encode and replace
    $init['formats'] = json_encode( $formats );

    return $init;
}
add_filter( 'tiny_mce_before_init', 'mkae_mce_awesome' );

First the settings have to be encoded to make them easy usable in PHP. Then we i add the class names (text--XXX) and remove the parts that cause the inline styling. At the end the array will be converted back.

Bonus: You can make the editor even more awesome by adding this line.

    $init['block_formats'] = 'Paragraph=p;Heading 2=h2;Heading 3=h3;Heading 4=h4';

This will prevent users from using <h1> headlines – the SEO nightmare…

I hope this will be useful for somebody. Improvements are welcome!

Also see https://wordpress.stackexchange.com/a/141539