Fallback to X in non-graphing apps since x and n are not usable.

apps/graph/app.cpp

@@ -86,4 +86,8 @@ Context * App::localContext() {
   return TextFieldDelegateApp::localContext();
 }
 
+const char * App::XNT() {
+  return "x";
+}
+
 }

apps/graph/app.h

@@ -38,6 +38,7 @@ public:
    * use a temporary local context (on the stack). That way, we avoid keeping
    * weird x values after drawing curves or displaying the value table. */
   Poincare::Context * localContext() override;
+  const char * XNT() override;
 private:
   App(Container * container, Snapshot * snapshot);
   Poincare::VariableContext<float> m_xContext;

apps/shared/text_field_delegate_app.cpp

@@ -22,7 +22,7 @@ AppsContainer * TextFieldDelegateApp::container() {
 }
 
 const char * TextFieldDelegateApp::XNT() {
-  return "x";
+  return "X";
 }
 
 const char * TextFieldDelegateApp::privateXNT(TextField * textField) {