Generating  with the form API
As an addition, just in case someone runs into the same trouble as I did - when using the #markup or #prefix/#suffix trick on a form's actions group, the submit callback function won't be called at all, unless a submit type element is present. My workaround was like this:

$form['actions']['submit'] = array
(
    '#type' =&gt; 'submit',
    '#value' =&gt; '',
    '#attributes' =&gt; array( 'style' =&gt; array( 'display: none' )), // hide the input field
    '#submit' =&gt; array( 'my_callback_for_the_form_submit' ),
    '#prefix' =&gt; '&lt;button type="submit" class="btn btn-primary"&gt;Add &lt;i class="fa fa-plus-square-o"&gt;',
    '#suffix' =&gt; '&lt;/i&gt;&lt;/button&gt;',
);


That way you can use custom HTML for submit action groups.