Import the outdated engines, but delete their pref so they show up as disabled. Clean up the old engines a little bit so they don't allocate unnecessarily and to not use syncCore.
* some minor hacking of the login progress to improve performance
* expanded the application exit sync dialog to be a general modal sync dialog that will display whenever appropriate (e.g. during initial setup, on manual "sync now" requests, and on application exit)
* added a progress meter and status messages to the modal sync dialog to provide users with a better understanding of what's going on, and to assist in debugging
* added ability to cancel a modal sync request, including on application exit. when cancel is clicked on by the user, it will attempt to cancel the sync at the next opportunity (i.e. before the next sync engine is processed)
This helps with code organization--all the logic for dealing with a particular data type is now in one place--and should also make it easier to write unit/regression tests.
