Circular reference problem

[Maciej Stachowiak <mjs eazel com>]

BonoboControl and BonoboControlFrame both hold refs on each other
while they are live. This leads to resource leaks if you try to do
proper refcounting on BonoboControlFrames instead of explicitly
destroying them. This used to be masked by the fact that a control was
destroyed if it was removed from its container, but now it is causing
a real resource leak in nautilus.

Possible solutions:

* Add a bonobo_control_frame_unbind_from_control API call so you can
  explicitly break the circular linkage.

* Have one of the two sides not hold a ref on the other.

* Ignore the problem, and document that ControlFrames be explicitly
  destroyed, but this solution sucks.

 - Maciej