Show the panel above the screenshield when locked

Track locked status and use it to provide a reduced version of
the panel in the locked screen. Accessibility, input sources and
volume menus are preserved, without the link to the control center.
Network, battery and user menu are reduced to pure indicators,
with no menu.
This is similar to the design but not exactly, because designers
in IRC said that network needs more analysis before exposing, and
because the design didn't account for a11y and IM (so the one menu
metaphor is not really appropriate).

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