Introduce shared SynqClient base synchronizer

With the addition of Dropbox as a sync target, we now have two synchroniser classes using the SynqClient library. For the time being, there are simply two different implementations which also means some code duplication.

We shall:

• Add an (abstract) SynqClientSynchronizer base class,
• which encapsulates common, SynqClient specific code and hence
• reduces the duplication.