MDL-32087 question preview: add a Save button.

Without this, there are some interactions with the question that could
only be tested by adding the question to a quiz, which was a pain.

lang/en/question.php

@@ -269,6 +269,7 @@ $string['questiontype'] = 'Question type';
 $string['questionuse'] = 'Use question in this activity';
 $string['questionvariant'] = 'Question variant';
 $string['reviewresponse'] = 'Review response';
+$string['save'] = 'Save';
 $string['saveflags'] = 'Save the state of the flags';
 $string['selectacategory'] = 'Select a category:';
 $string['selectaqtypefordescription'] = 'Select a question type to see its description.';

question/preview.php

@@ -252,6 +252,8 @@ echo $quba->render_question($slot, $options, $displaynumber);
 echo '<div id="previewcontrols" class="controls">';
 echo '<input type="submit" name="restart"' . $restartdisabled .
         ' value="' . get_string('restart', 'question') . '" />', "\n";
+echo '<input type="submit" name="save"' . $finishdisabled .
+        ' value="' . get_string('save', 'question') . '" />', "\n";
 echo '<input type="submit" name="fill"' . $filldisabled .
         ' value="' . get_string('fillincorrect', 'question') . '" />', "\n";
 echo '<input type="submit" name="finish"' . $finishdisabled .