Add a `default_featured_apps` boolean option which controls whether the
default featured apps list (`org.gnome.Software.Featured.xml`) is
installed. On most distros, it makes sense to install this (and the
option defaults to `true`), but some distros will want finer control
over their featured apps (or to override the entire list). Those distros
should use `-Ddefault_featured_apps=false` and set the default value of
`external-appstream-urls` in GSettings to point to a relevant override
appstream file which sets the desired featured apps.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #692