libsrtp.recipe: Fix building on targets/hosts other than Linux

Ensure that the correct versioned library is generated for each platform. Not
having a versioned library also causes `cerbero package` to skip the shared
library.
recipes/custom.py: Add support for SourceType.TARBALL

custom.py was assuming that all recipes using it are of type GIT or GIT_TARBALL.
This is a hacky attempt (just as hacky as custom.py itself) to make it support
tarballs as well.

Note that cerbero already has really hacky support for recipe names that don't
match the tarball unpacked directory name, and that's required to make this work.
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: icu.recipe

Cross-compilation is complicated, so has been left alone for now
harfbuzz.recipe: Now that we have ICU, we can actually build with ICU support
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