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/node/context.h

@@ -20,6 +20,7 @@ class PeerLogicValidation;
 namespace interfaces {
 class Chain;
 class ChainClient;
+class WalletClient;
 } // namespace interfaces
 
 //! NodeContext struct containing references to chain state and connection
@@ -40,7 +41,11 @@ struct NodeContext {
     std::unique_ptr<BanMan> banman;
     ArgsManager* args{nullptr}; // Currently a raw pointer because the memory is not managed by this struct
     std::unique_ptr<interfaces::Chain> chain;
+    //! List of all chain clients (wallet processes or other client) connected to node.
     std::vector<std::unique_ptr<interfaces::ChainClient>> chain_clients;
+    //! Reference to chain client that should used to load or create wallets
+    //! opened by the gui.
+    interfaces::WalletClient* wallet_client{nullptr};
     std::unique_ptr<CScheduler> scheduler;
     std::function<void()> rpc_interruption_point = [] {};