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/init_test_fixture.cpp

@@ -10,7 +10,7 @@
 
 InitWalletDirTestingSetup::InitWalletDirTestingSetup(const std::string& chainName) : BasicTestingSetup(chainName)
 {
-    m_chain_client = MakeWalletClient(*m_chain, *Assert(m_node.args), {});
+    m_wallet_client = MakeWalletClient(*m_chain, *Assert(m_node.args), {});
 
     std::string sep;
     sep += fs::path::preferred_separator;