Resolve "Use QtKeychain instead KWallet directly"

This migrates from KWallet to QtKeychain. The only side effect is that effectively the passwords previously saved to a vault will need to be either:

1. typed-in again

2. have their keys adjusted in operating system's keychain/vault software. For example, on macOS, we previously saved under the name "kdewallet", which was KWallet's default, but now we save to "org.kde.kmymoney".

kmmkeychain.cpp/h is loosely based on https://github.com/frankosterfeld/qtkeychain/tree/master/TestAppExample

Closes #46 (closed)