Let the keychain return new creds during async save.

The credentials are asynchronously saved. The credentialstore now returns
the creds even if the saving has not yet finished but is running, for that
the new state AsyncWriting was added. That makes the start of the sync
after the password was changed more reliable.
Let the folderman create a new sync directory.

If a sync directory exists its moved to a backup path and created new.
That allows to start a sync "from scratch".
Added a checkbox "start sync from scratch".
Handle password change correct, add "sync from scratch".

The setup wizard now destinguishes if there was already a configuration
or not. If yes, it's going to password-change mode. Also added a checkbox
that allows to start a sync from a cleared local directory.