Introduce (very) basic, incomplete version of CoreConnection

Introduce (very) basic, incomplete version of CoreConnection

This class encapsulates the client-side connection to the core and replaces
both ClientSyncer and CoreConnectDlg. Having this neatly encapsulated will allow
for a much easier way to do, say, reconnects or tracking the connection state.

This is a first preliminary version that can only sync to a hardcoded account.