Use QtKeychain instead KWallet directly

Using KWallet directly imposes limitations on which secure key enclaves we can use. KWallet support for non-Unix platforms is limited and, for example, macOS KeyChain backend is currently broken.

What was suggested is that KMyMoney migrates to QtKeychain, which abstracts the key storage depending on the platform. This would add support not only for macOS, but also for Windows and Gnome Keyring.