Enable the GUI to also use the load_on_startup feature.
Wallets loaded in the GUI always have load_on_startup=true.
When they are unloaded from the GUI, load_on_startup=false.
To facilitate this change, UpdateWalletSetting is moved into the wallet
module and called from within LoadWallet, RemoveWallet, and
Createwallet. This change does not actually touch the GUI code but
rather the wallet functions that are shared between the GUI and RPC.
This maintains a persistent list of wallets stored in settings that will
automatically be loaded on startup. Being able to load a wallet automatically
on startup will be more useful in the GUI when the option to create wallets is
added in #15006, but it's reasonable to expose this feature by RPC as well.
