cerbero/packages/package.Package: Aggregate the files list in packages correctly

Don't override the files included from a previously-specified recipe.
Instead, append to it.

Technically, you can do the same thing as:

  files = ['libsoup:libs', 'libsoup:devel']

By doing:

  files = ['libsoup:libs:devel']

But this is undocumented, mostly unused, and there's no reason why the first one
shouldn't also work.
New recipes: libnice.recipe, libnice-static.recipe

Also add to the gstreamer-1.0-net package
New recipe: libsusrsctp.recipe
New recipe: gnome-js-common.recipe
New recipe: javascriptcoregtk.recipe

Lots of hacks to build only JSC and not WebKit-GTK+
More hacks to cross-compile on Android
New recipe: seed.recipe
New recipe: openwebrtc-gst-plugins.recipe
New recipe: openwebrtc.recipe