FEATURE: Invoke onSubmit callback on pages

Form Pages are an instance of `AbstractSection` but their `onSubmit()`
callback was never invoked by the Form Framework.

Resolves: #148

Classes/Core/Runtime/FormRuntime.php

@@ -280,13 +280,18 @@ protected function mapAndValidatePage(Page $page)
             }
         };
 
+        $pageFormValues = [];
         foreach ($page->getElementsRecursively() as $element) {
             $value = Arrays::getValueByPath($requestArguments, $element->getIdentifier());
             $element->onSubmit($this, $value);
 
-            $this->formState->setFormValue($element->getIdentifier(), $value);
+            $pageFormValues[$element->getIdentifier()] = $value;
             $registerPropertyPaths($element->getIdentifier());
         }
+        $page->onSubmit($this, $pageFormValues);
+        foreach ($pageFormValues as $elementIdentifier => $value) {
+            $this->formState->setFormValue($elementIdentifier, $value);
+        }
 
         // The more parts the path has, the more early it is processed
         usort($propertyPathsForWhichPropertyMappingShouldHappen, function ($a, $b) {