utils: Use Libsoup directly for HTTP(s) downloads

This currently applies to place icons from Nominatim.
Provide an AppData file

This is important for Maps to appear (nicely) in gnome's software
center. More details:

http://blogs.gnome.org/hughsie/2013/08/29/gnome-software-center-and-you/

Based on a patch from Andreas Nilsson <anilsson@redhat.com>.

https://bugzilla.gnome.org/show_bug.cgi?id=707125