[QA] Client ask for credentials even if logged out before

[gabi18]

Seen with 2.9.0beta1 on Win10 and openSUSE 15.2

Steps done:

• click on "Account" button and 'Log out'
• quit ownCloud client
• restart client

Result:

• the client ask for credentials (either on browser page or with a popup)

Expected result:

• the client shouldn't ask for login credentials if the use has logged out before

TODO:
Save the logged out state to the settings
AccountState::signOutByUi