Fix thumbnails sidebar resizing issues with GTK3

Closes #58
https://github.com/mate-desktop/atril/issues/58

https://git.gnome.org/browse/evince/commit/shell/ev-sidebar-thumbnails.c?id=a621f05b42468606c8a04446d54b637ecdd05c9b
https://git.gnome.org/browse/evince/commit/shell/ev-sidebar-thumbnails.c?id=1e88e7fd7630c28453475e48b98941ce11b130a6