desktop-shell: Map input panel exactly once

When the input panel surface gets something committed to it, we have
content and thus the surface has become mapped. Do this from within the
input-panel surface committed handler.

When we want to show the input panel, this is when we map the view. Do
this exactly once as well, and make sure that we don't attempt to map
the view with an unmapped surface.

This regularises the logic to be the same as almost all the other
special surface users.

Signed-off-by: Daniel Stone <daniels@collabora.com>