miniobject: Add parent pointers to the miniobject to influence writability

miniobject: Add parent pointers to the miniobject to influence writability

Every container of miniobjects now needs to store itself as parent in
the child object, and remove itself again at a later time.

A miniobject is only writable if there is at most one parent, and that
parent is writable itself, and if the reference count of the miniobject
is 1.

GstBuffer (for memories), GstBufferList (for buffers) and GstSample (for
caps, buffer, bufferlist) was updated accordingly.

Without this it was possible to have e.g. a bufferlist with refcount 2
in two places, modifying the same buffer with refcount 1 at the same
time.

https://bugzilla.gnome.org/show_bug.cgi?id=796692