Fix the missing CSS classes in Contao 5.3 (see #99)

[qzminski]

Fixes #99

[qzminski]

@fritzmg do you think this solution is good enough for the time being? :)

[fritzmg]

Not sure, do the Twig templates work at all in 5.3? What I mean is: does

contao-mp_forms/contao/templates/form_mp_form_pageswitch.html.twig

Line 4 in 6680f77

<button type="submit" name="{{ this.name }}" value="continue">{{ this.slabel }}</button>

get the correct name and label?

[qzminski]

Yes, that works. The problem was that the data was not correctly "transferred" from this Twig template to the form_row.html5 legacy template.