UI freeze break request for eog

Hi!

This is a really tiny UI freeze break request for Eye of GNOME.
While testing the new UI Allan Day found that the margin to the sides of
the revamped metadata sidebar are pretty small and suggested increasing
the margins.

It would be nice if we could get this in as the broader margins indeed
looks nicer (esp. at the window border) and improve the separation
between image and sidebar.

I prepared a Bugzilla ticket that contains comparision images and the
necessary two lines of code:

https://bugzilla.gnome.org/show_bug.cgi?id=746105

Documentation is unaffected by this change as we unfortunately didn't
manage to update the docs for the new UI in this cycle.

Regards

Felix