ScreenShield: show notifications in the locked screen

ScreenShield: show notifications in the locked screen

Track screen lock status in the message tray, and filter banner
notifications. The message tray is completely hidden when the screen is
locked, but exceptions can be made for individual transient notifications,
such as shell messages and the on screen keyboard.
Non transient sources are shown in the middle of the lock screen. Resident
notifications (such as those from Rhythmbox) are shown in full, while
persistent ones are displayed as icon and message count.

https://bugzilla.gnome.org/show_bug.cgi?id=619955