ScreenShield: fix positioning of background with multimonitor

ScreenShield: fix positioning of background with multimonitor

Previously, we would create one StBin per monitor, but each was positioned
at 0,0 and sized as the screen, so they would overlap and draw the box shadows
on top of the other backgrounds.
Instead, we need to size appropriately the bin, and then we need to position
the actual MetaBacgroundActor at 0,0, so add a flag to BackgroundManager
for this.
Also, get rid of MetaBackgroundGroup, they do nothing because the screenshield
is not a descendant of the MetaWindowGroup and because the widget in between
blocks the propagation of the visible region. At the same time, use a
widget, not a bin, because StBin requires you to set .child, not call add_child().

https://bugzilla.gnome.org/show_bug.cgi?id=694394