SystemMenu: Fix bad behavior in shutdown dialog

The selected option was stored in a captured stack variable which was
long gone by the time we looked at it, so this dialog didn't really
behave the way you'd expect. Put it in a member instead. :^)

Applications/SystemMenu/PowerDialog.h

@@ -36,4 +36,6 @@ public:
 private:
     PowerDialog();
     ~PowerDialog();
+
+    int m_selected_option { -1 };
 };