This commit changes gtk_extended_layout_get_desired_size() for
per dimension variants. Furthermore this commit reverts the actions
done in size-groups for now as it needs a different approach.

The natural width/height parameters added to aux_info have been changed
for a per width cache for heights and a per height cache for widths.

gtk-demo is still working, currently sizegroups are not taken
into account as mentioned above - size groups need to be alerted both
when the widths and heights are updated independantly and then that
information needs to repropagate also to other extended layout implementors.