screenShield: Show detailed notification data if notifications hint it

screenShield: Show detailed notification data if notifications hint it

Allow notifications to set a x-gnome-privacy-scope hint, with values in
['system', 'user']. If all the notifications in a particular source hint
that their privacy scope is ‘system’, don’t hide the notification
details on the lock screen.

This is aimed at fixing the particular case of power notifications: they
contain information which is not private to the user (it relates to the
system: battery state or AC state, which is obvious to anyone who can
see the machine), so hiding the details of a power management
notification when the screen is locked is pointless.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
https://gitlab.gnome.org/GNOME/gnome-shell/issues/726