Do not activate network on silentinstall

Closes #120 1.3 crashes with option "silentinstall"

(reason it crashes is actually that we are calling _qpd->hide()
when downloading the list of the Internet is complete, while _qpd
is not created on a silentinstall.
But no need for the network on a silent install anyway)
Added warning about the behaviour of autoboot.txt
Hide local image if newer network image is available

This incorporates commits c260fd6, d248994 and 9e845cb from the
dev branch, as well as related tweaks and fixes.
Offline and online OSes now co-exist much more harmoniously.
Display blank 'source' icon if OS is un-installable

Half-way towards fixing #143
Revert "Add small note about bug #120"

This reverts commit e21b0ea3f998e99ae247ad995dcac2b8dc3e3b64.

14bdcc7 fixes the bulk of #120 (I'd simply left it lingering on the dev branch)
Change default URL launched by "Online help" web-browser
NOOBS v1.3.10