Add support for the CSS :first-child and :last-child properties to
StContainer, and thus to all containers in St and ShellGenericContainer.
The internal ordering of the container's children is used to determine
the child to which to attach the pseudo class, not the children's
positions. This means that containers where positions can differ from
the ordering (ShellGenericContainer / StGroup) may behave unexpectedly,
so some caution is required.
https://bugzilla.gnome.org/show_bug.cgi?id=625316
Add StContainer, which implements the ClutterContainer interface based
on the container methods in st-private and make the existing containers
subclass it.
https://bugzilla.gnome.org/show_bug.cgi?id=613907
