Wizards: let the page use wizard values

For example it will be used to get the
wizard directory to show icons in fields later.

Change-Id: I6a243a67a89e4a0133550a61b870d771147667f1
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/jsonwizard/jsonfieldpage.h

@@ -125,6 +125,8 @@ public:
 
     Utils::MacroExpander *expander();
 
+    QVariant value(const QString &key);
+
 private:
     static QHash<QString, FieldFactory> m_factories;