gthumb crashes when attempting to play a video

Gthumb crashes when attempting to play a video under special
circumstances.

How to reproduce:
- start gthumb in a terminal
- disable all extensions besides "Audio/Video support" (especially "Burn
CD/DVD" must be turned off)
- restart gthumb as requested
- navigate to a directory containing a video
- click on a video

Current result:
- gthumb is closed with the following error message:

gthumb: symbol lookup error:
/usr/lib64/gthumb/extensions/libgstreamer_tools.so: undefined symbol:
gst_is_video_overlay_prepare_window_handle_message

Comments:
- with the port to gstreamer-1.0, the offending function call was
introduced
- however, that function is provided by libgstvideo-1.0.so which is not
linked
to the gstreamer_tools extension

- that it works for some people and but not for others is caused by a
different
selection of enabled extensions: once "Burn CD/DVD" is enabled, it
suddenly
starts to work because that extension is linked against
libbrasero-burn3.so
which is linked against libgstvideo-1.0.so

- the correct solution seems to link libgstvideo-1.0.so to
libgstreamer_tools.so

[bug #706343]