Add support KeePassXC backend to QtKeychain

[MurzNN]

KWallet development in KDE seems stopped, so some users move to other password managers, most popular is KeePassXC. Will be good to have support for KeePassXC password storage in QtKeychain.

[T-bond]

I think this is not longer required, as KeepassXC can function as a libsecret provider.

Maybe if it is possible, invoke the KeepassXC dialog to unlock the store, when some program requests credentials, but the keystore is locked. I think this is the main issue currently with Nextcloud.