With more and more applications using KGAPI (like KDE PIM) and sharing the same API keys, it's no longer possible for them to each store the accounts separately - each app that has different scopes but uses the same API keys invalidates the other's tokens, which ends up in endless auth fight.

To handle that we would either need to have lots of keys, or we can have a centralized piece of code that handles storing the tokens in KWallet and cleverly merges the scopes needed by mutliple apps using the same API key into a single account.

It can also take care of authenticating new accounts, which would simplify the existing code a bit.