JsonWizard: Check widget visibility when determining if it's mandatory

The wizard is unable to advance when a mandatory field is hidden. Assume
a hidden widget is not mandatory.

Change-Id: I19bdc1a661d91b75151d3aec34ad9d584002a556
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/jsonwizard/jsonfieldpage.cpp

@@ -955,7 +955,7 @@ bool JsonFieldPage::isComplete() const
                 showError(message);
                 hasErrorMessage = true;
             }
-            if (f->isMandatory())
+            if (f->isMandatory() && !f->widget()->isHidden())
                 result = false;
         }
     }