please backport the bug fix for the "crash on exit problem"

original bug report: https://bugzilla.gnome.org/show_bug.cgi?id=621597

original description:

F-Spot 0.6.2 crashes very frequently on exit. It is not 100% reproducible, but
I see the problem probably in 20% of the tests.

How to reproduce:
- open f-spot
- just use it (the following steps may help to reproduce the issue)
* scroll in the browse view
* double click one picture
- exit f-spot (e.g. using the "X" button of the f-spot window)

The same issue was also reported by some f-spot users in Fedora:
https://bugzilla.redhat.com/show_bug.cgi?id=596601

According to the output during the crash the problem may be related to threads:

ERROR:threads.c:1099:ves_icall_System_Threading_Thread_Thread_free_internal:
assertion failed: (!this->abort_exc && !this->abort_state_handle)
Stacktrace:

  at (wrapper managed-to-native) System.Threading.Thread.Thread_free_internal
(intptr) <0x00004>
  at (wrapper managed-to-native) System.Threading.Thread.Thread_free_internal
(intptr) <0x00004>
  at System.Threading.Thread.Finalize () <0x00017>
  at (wrapper runtime-invoke) object.runtime_invoke_virtual_void__this__
(object,intptr,intptr,intptr) <0x00046>

Please also note, that it looks like that this issue got introduced with 0.6.2.
I have not seen it before (or it happened with a much less frequency).