ShmPool returns QWeakPointer<Buffer> instead of Buffer*

The Buffers are exclusively hold by ShmPool. The user of a Buffer is not
supposed to delete it as a no longer used Buffer should be reused by the
ShmPool.

To make it obvious that the ownership of the pointer is not passed to the
user the return type is changed to QWeakPointer. This also allows the
ShmPool to destroy Buffers as needed.