Display the model in the control panel About this computer section

The model is read from gconf
/desktop/sugar/extensions/aboutcomputer/hardware_model key
if not available, is not displayed.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
About this computer: Display secondary licenses if needed

A directory is configured, where deployments, can add text files
with the licenses to display.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
About computer: show when was the last update on system packages

The mtime in the file /var/lib/misc/last_os_update.stamp is used
to know when was the last update.

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>
Reorganize the code to avoid duplications

Signed-off-by: Gonzalo Odiard <gonzalo@laptop.org>