Improved migration, file-based StoreUUID, Mac fixes.

[ADDED]     Migration to file-based StoreUUID.
[IMPROVED]  Cleaner, more modular, migration code.
[FIXED]     Don't hook activation to show password window: too annoying when focus shifts.
[FIXED]     Don't set the active user when just importing ubiquity changes, it causes a log-out.
[ADDED]     Mac: Ability to rebuild iCloud.
[REMOVED]   Mac: wasRunning logic to avoid password window appearance on first activation.
[FIXED]     Mac: Retrieve siteName from main thread.