actor: Call show() on set parent conditionally

actor: Call show() on set parent conditionally

This commit adds a further conditional check for calling
clutter_actor_show() when adding a child to an actor. We cannot
unconditionally change the value of the show-on-set-parent property like
the original solution of commit 81b19a78f5e02ae8ffd9235054d80b172496415b
as that breaks the document invariant that show-on-set-parent will be
changed iff an actor is without a parent.

The new ADD_CHILD_SHOW_ON_SET_PARENT flag is part of the default and
legacy flags, thus retaining the default behaviour when adding a child;
the flag is not passed when reordering the list of children, which means
we ignore the state of the show-on-set-parent property.

The conformance test suite fully passes, including the newly added test
to verify that changing the paint order does not trigger visibility.

https://bugzilla.gnome.org/show_bug.cgi?id=674510