Switch to the Attica framework

Summary:
Switch from the old version of Attica, libattica-ocsclient, to the
frameworks version of Attica, adapting the code in the following ways:

• manually load the providers using ProviderManager
• switch to the currently working provider, i.e. the kde-look.org one
• deal with non-const methods of Provider by copying it locally
• use the different API of jobs
• manually load the avatars (always with KIO), since Attica does not do that

Sadly, there is no kde/opendesktop provider that seems to provide
information on users at this time, so most of this is not tested much.
(But then, for the same reason the old code is not usable either.)

Test Plan: Builds fine

Reviewers: Amarok, heikobecker

Reviewed By: heikobecker

Subscribers: amarok-devel

Differential Revision: https://phabricator.kde.org/D13180