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.
e4f4350471