In StBin, StBoxLayout, and StTable, if a child has a potential
allocation that is larger than its preferred size, we give it its
preferred size instead. However, the corresponding
get_preferred_height/width methods were not making the same
assumption, which meant that if we had more width than the widget
wanted, we would allocate it its preferred width, but with the height
that corresponded to the larger width.
Fix this by defining new helpers _st_actor_get_preferred_width() and
_st_actor_get_preferred_height() and using them everywhere. Also, make
StBin and StTable use _st_allocate_fill() rather than having
nearly-identical duplicate copies of the code.
https://bugzilla.gnome.org/show_bug.cgi?id=609848
