This should make both the runtime and the sdk correctly show up in gnome-software.
I am not sure what to do with the license field; I've left it as
GPL-2.0+ for now, matching what the freedesktop and gnome runtimes are
doing, but it's clearly not correct (we include many libraries with many
different licenses, not just GPL-2.0+). I've left a FIXME in the appdata
files for now.