libview: setting annotation window icons sizes

After a gtk update, the sizes of the icons on the
annotation window (close and resize handles) are
48x48. This patch fixes this by using GtkIconTheme
to force the loading of the icon on a different
size than the file.

https://bugzilla.gnome.org/show_bug.cgi?id=735110
libview: removing ev_annotation_window_get_icon_size

With the modifications on how the size of the
icon is set, this function is no longer used.