Refactor Account in preparation for multi account feature

Steps
* libsync/Account got split into libsync/Account (url, creds, qnam, ssl) and gui/AccountState (connectivity state, signed out?)
* State management functionality from Application moved into AccountState
* Folder, SyncEngine and OwncloudPropagator are tied to an Account, which makes it possible to create Folders for different Accounts

There's still a bunch of work left: All uses of AccountManager should be looked at and probably removed.

Eventually Accounts could be owned by AccountStates (for the gui case).

Testing needs:
* The setup wizard for 'start from scratch' and 'skip folder setup' cases
* Multiple folders
* General stability

There should have been no user-visible changes.