unattended-setup-box: Add missing private keywords

https://bugzilla.gnome.org/show_bug.cgi?id=734675
unattended-setup-box: Detect internet if needed

This is needed to take the needs-internet param of Osinfo.InstallScript
into account for the express installation.

https://bugzilla.gnome.org/show_bug.cgi?id=734675
unattended-setup-box: Show hint if internet needed

https://bugzilla.gnome.org/show_bug.cgi?id=734675
unattended-installer: Take needs-internet into account

The needs-internet param from Osinfo.InstallScript signals that an
express installation needs an internet connection in order to run
without user intervention.

https://bugzilla.gnome.org/show_bug.cgi?id=734675

Conflicts:
	src/unattended-installer.vala