Credentials: Namespace windows cred keys

[ckamm]

The application name is prepended to the key. QtKeychain doesn't
do that automatically on the platform.

See #6125

@SamuAlfageme I'd like to ask you to redo the 2.3 -> 2.4 upgrade tests after this patch hits. It's expected that 2.4alpha credentials won't be found or deleted.

[guruz]

It looks good. We could merge it but let's think about this:

Should we have QtKeychain as git submodule #1416 and fix it directly in the QtKeychain fork and then attempt to upstream this to QtKeychain?

Hm.. @ikushn-s mentioned though that this sucks a bit for QtKeychain users updating to a newer QtKeychain with suddenly different keynames..
CC @frankosterfeld
Opinions?

[ckamm]

Fixing it upstream would be just as well. I agree that there'd need to be a flag to toggle between old and new behavior to make migration possible. We'd still need the migration code - we'd just save on composing the key ourselves. The main advantage would be alerting other users of the library and, possibly, changing the default behavior.

@guruz I'd merge this as-is. If there's an update to QtKeychain we can update to using that with minor changes.