Notify the GUI that the keypool has changed to set the receive button

Whenever the keypool changes (new keys generated, new seed set,
keypool runs out, etc.), notify the GUI that the keypool has changed. The
receive button can then be enabled and disabled as necessary.

src/interfaces/wallet.cpp

@@ -488,6 +488,10 @@ public:
     {
         return MakeHandler(m_wallet.NotifyWatchonlyChanged.connect(fn));
     }
+    std::unique_ptr<Handler> handleCanGetAddressesChanged(CanGetAddressesChangedFn fn) override
+    {
+        return MakeHandler(m_wallet.NotifyCanGetAddressesChanged.connect(fn));
+    }
 
     std::shared_ptr<CWallet> m_shared_wallet;
     CWallet& m_wallet;