This ports ShellGenericContainer to St. There is still more work that
could be done here (notably redoing skip_paint in terms of child_meta),
but this gets us most of the way there.

I started fixing up some of the existing hackarounds in the code, mostly
just to test out the rewrite. That's in the last patch, but it's incomplete,
and not ready to go yet. I'm interested what people think of the cssBind()
function though. (I actually mostly wrote it for appDisplay.js, but then
didn't get around to porting that yet.)

I'm assuming we don't want GenericContainer to move to St, since it's
"mainly a workaround for the current lack of GObject subclassing + vfunc
overrides in gjs". However, it might make sense to create St.Group (an
St.Widget-based Clutter.Group) and then have Shell.GenericContainer be
a subclass of that? I think there are places where either we have a
Clutter.Group inside an St.Bin, or else we have an St.BoxLayout with
the majority of the contents fixed-positioned rather than box-layout-ed.
So St.Group might be useful...