Instead of setting the size on each child, set it on the parent grid
because only one of those children are visible at any given point.
We are cheating a bit by ignoring the fact there is a second child
which is always shown -- the button box. This will make the other
children lose some height and the dialog will be smaller. However it is
still too big for smaller screens.
https://bugzilla.gnome.org/show_bug.cgi?id=708943
Show a message to the user when the Pictures source is empty, inspired both
in design and widget layout by the EmptyResultsBox from gnome-documents.
https://bugzilla.gnome.org/show_bug.cgi?id=630892
