Add IsLegacy to CWallet so that the GUI knows whether to show watchonly

src/interfaces/wallet.h

@@ -266,6 +266,9 @@ public:
     // Remove wallet.
     virtual void remove() = 0;
 
+    //! Return whether is a legacy wallet
+    virtual bool isLegacy() = 0;
+
     //! Register handler for unload message.
     using UnloadFn = std::function<void()>;
     virtual std::unique_ptr<Handler> handleUnload(UnloadFn fn) = 0;