Add surface previously unmapped to the layer

This makes set_position() do only what it advertise (that is, setting
the position of a surface).

In configure_static_surface() we add back to the layer surfaces that
were unmapped (such as the AppChooser one).
Set background surface position

This will position background properly on multi display setups.
Bind Ctrl+Alt+Backspace to terminate display

Useful when nothing else can tear the compositor down.