This will help moving the SyncEngine construction in the constructor
and allow moving functionalities from Folder to SyncEngine or its
delegated objects.
The creation doesn't need to be separated from the SyncEngine anymore.
This allows the SyncEngine to be created in fewer steps if we want to
use it in tests.
This moves most of the direct csync code from Folder into the SyncEngine.
The exclude file logic for the context has been wrapped using the
existing ExcludedFiles class as well.
Introduce a global ExcludedFiles instance to avoid loading the global
exclude lists several times.
One could still add per-folder exclude lists by checking these after
the global ones.
