Log Message:
xfce4 & xfce4-extras: move gnome-themes-standard back to xfce4

gnome-themes-standard had been moved to extras to try and help 32-bit
ARM builds generate a complete base DE, because of librsvg breakage.
With Xfce 4.16 having various components that now use CSDs, we really
must have this package as part of the basic install. (And, in any case,
32-bit ARM builds have different dependent package breakages now.)