In Jessie, Seahorse has moved to the Utilities GNOME menu section.
Add helper for more robust GNOME window waiting.

That is, we make sure that the expected X button can be found
somewhere right of the matched image. This will be useful in cases
where there's a non-window title part of the screen that matches as
well...
Reliably wait for Seahorse's window.

It turns out the GNOME applications menu entry and the window title
looks exactly the same, so if we wait for the window immediately after
we've started it throught the menu, the menu entry can become a false
positive.
Update the test suite's Seahorse images vs Jessie.
Install gstreamer1.0-libav.

So e.g. Totem can play h264 encoded videos (like in the automated test
for Totem).
Remove Totem test steps blocked by Debian bug #768515.

Once fixed, this commit should be reverted.
Update some images for the Totem tests vs Jessie.
Use wait_*_gnome_window() helpers everywhere applicable for added reliability.