background: Make whole dialog visible on small screens

When using a small screen, make sure that the whole of the dialogue
is visible, including the confirmation buttons. This means that the
dialogue will be maximised when on smaller screens (just like its
parent), and that we won't set a number of columns for the icon
view.

We will also stop making the dialogue much bigger than the
Settings window itself.

https://bugzilla.gnome.org/show_bug.cgi?id=692495