Basic infrastructure for file transfers

Basic infrastructure for file transfers

This introduces the Transfer class as well as TransferManager and the
specialized versions for client and core. It also adds the needed accessors
in Client and CoreSession as well as the SyncableObject stuff to make it all
work together.

To make something happen, a Transfer needs to be created on the core side with
the appropriate attributes; it needs then to be synchronized and handed over
to CoreTransferManager::addTransfer(). This will trigger the creation of the
appropriate Transfer object on the client side, adding this to the
ClientTransferManager as soon as synchronization is complete (meaning, the
transferAdded() signal will be fired with a fully synchronized Transfer object).

Note that the list of transfers is not synchronized (yet?), so there won't be a
transfer history on re-sync.