UI for backports usage

[Milan]

There's obviously a need for a new infrastructure/UI. Since GDebi and
apt-url are to be disabled in future versions (which I approve of), we
should find a way to allow the user to select trusted repositories and
easily install software from there, either on a
replace-alla-installed-packages basis or on a ponctual one.

Maybe apt-url could be modified only to accept trusted repos. Else, we
would need a new graphical tool, or new features in "Add/Remove
applications".