wallet.use_change is a weird preference using which a user can disable
sending new change to change addresses.
However the setting can be toggled at ~any time; and the user might
have pre-existing balance on change addresses, which we should not hide.
(this somehow escaped attention before, as most objects usually don't have multiple instances,
unless multiple wallets are open at the same time.)
Also, move all signal declarations, class constants and variables to the top of class definitions.
fixes https://github.com/spesmilo/electrum/issues/7919
In the past, when creating payment requests, we keyed them by on-chain address,
and set/saved the msg of the request as label for the address.
Many places in the code were calling wallet.get_label(addr) with the expectation that
relevant payment requests are found and their message/description (if any) is considered.
wallet.get_label(key) is now made private, and instead the explicit non-polymorphic
wallet.get_label_for_{address,rhash,txid} alternatives should be used.
