st: Add "Since" blurbs for StIconTheme's introspectability
StIconTheme
was added to public introspected types, but since St
is
not versioned (like e.g. Clutter
) it can't use Since
annotations in
a reasonable way.
Add a blurb at the bottom of public member documentation, noting when
StIconTheme
became available for GNOME Shell extensions; the primary
consumer of introspection.