refactor: Move wallet methods out of chain.h and node.h

Add WalletClient interface so node interface is cleaner and don't need
wallet-specific methods.

The new NodeContext::wallet_client pointer will also be needed to eliminate
global wallet variables like ::vpwallets, because createWallet(), loadWallet(),
getWallets(), etc methods called by the GUI need a way to get a reference to
the list of open wallets if it is no longer a global variable.

Also tweaks splash screen registration for load wallet events to be delayed
until after wallet client is created.

src/wallet/test/wallet_test_fixture.cpp

@@ -11,5 +11,5 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName)
     bool fFirstRun;
     m_wallet.LoadWallet(fFirstRun);
     m_chain_notifications_handler = m_chain->handleNotifications({ &m_wallet, [](CWallet*) {} });
-    m_chain_client->registerRpcs();
+    m_wallet_client->registerRpcs();
 }