MOC saving improvements, Mac app activation, loading overlay, USM update.

[MOVED]     iOS code from MPAppDelegate_Shared to MPiOSAppDelegate.
[FIXED]     Perform MOC saving in MOC perform blocks.
[FIXED]     Mac: Activate app when showing password window and not active.
[FIXED]     Hide overlay while corruption dialog is up.
[FIXED]     Hide corruption dialog when reloading store.