Added base class to hold alignment and overall size request information
while itterating over the size requests of various rows of a GtkTreeModel,
updated GtkCellArea/GtkCellAreaBox classes accordingly.
Added the child list to GtkCellAreaBox, added _pack_start() and
_pack_end() apis to GtkCellAreaBox since they are appropriate there
and implemented GtkCellLayoutIface to override the _pack_start()/end()
methods (since the base GtkCellArea class simply forwards these apis
to the generic ->add() api on the base class).
Now GtkCellArea handles attribute connections in the base class,
subclasses only need to add/remove the renderers, render them,
do geometry and handle events.
Now GtkCellArea provides a generic way of applying attributes
from a GtkTreeModel/GtkTreeIter, GtkCellArea bookkeeps a hashtable
of GtkCellLayoutDataFunc's and completely abstracts the applying
of data to cells... GtkCellArea implementations need only to bookkeep
the added renderers and attributes (probably we can abstract the
attribute bookkeeping in the base class as well).
Things starting to take a good and practical shape.
