MDL-47410 form: Add data-fieldtype attribute to checkbox inline element

Includes the data-fieldtype attribute to the checkbox inline form
element. This element does not extend the parent inline element
template due to its specific nature and therefore this attribute was
missing.

lib/form/templates/element-checkbox-inline.mustache

@@ -1,4 +1,4 @@
-<label class="form-check {{#error}}has-danger{{/error}} fitem {{#advanced}}advanced{{/advanced}} {{{element.extraclasses}}}">
+<label data-fieldtype="checkbox" class="form-check {{#error}}has-danger{{/error}} fitem {{#advanced}}advanced{{/advanced}} {{{element.extraclasses}}}">
 {{^element.hardfrozen}}
     {{#element.frozen}}
         <input type="hidden" name="{{element.name}}" value="{{element.frozenvalue}}">