Merge #18160: gui: Avoid Wallet::GetBalance in WalletModel::pollBalanceChanged

0933a37078e1ce3a3d70983c3e7f4b3ac6c3fa37 gui: Avoid Wallet::GetBalance in WalletModel::pollBalanceChanged (João Barbosa)

Pull request description:

  Each 250ms the slot `WalletModel::pollBalanceChanged` is called which, at worst case, calls `Wallet::GetBalance`. This is a waste of resources since most of the time there aren't new transactions or new blocks. Fix this by early checking if cache is dirty or not.

  The actual balance computation can still hang the GUI thread but that is tracked in #16874 and should be fixed with a solution similar to #17135.

ACKs for top commit:
  hebasto:
    ACK 0933a37078e1ce3a3d70983c3e7f4b3ac6c3fa37, I have not tested the code, but I have reviewed it and it looks OK, I agree it can be merged.
  jonasschnelli:
    utACK 0933a37078e1ce3a3d70983c3e7f4b3ac6c3fa37
  instagibbs:
    ACK 0933a37078e1ce3a3d70983c3e7f4b3ac6c3fa37
  ryanofsky:
    Code review ACK 0933a37078e1ce3a3d70983c3e7f4b3ac6c3fa37, but I would prefer (not strongly) for #17905 to be merged first. This PR can be simpler if it is based on #17905, so tryGetBalances can just be left alone instead of changing into to a more complicated tryGetBalancesIfNeeded function, and then getting changed back later when we want to optimize it out.
  jonatack:
    ACK 0933a37078e based primarily on code review, despite a lot of manual testing with a large 177MB wallet.

Tree-SHA512: 18db35bf33a7577666658c8cb0b57308c8474baa5ea95bf1468cd8531a69857d8915584f6ac505874717aa6aabeb1b506ac77630f8acdb6651afab89275e38a1

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