The rationale here is that every cell in an area needs to have space
reserved around it, requests have to be fully margin inclusive...
cells need to have the full size fed as the "background area" and
the "cell area" has margins removed... This will be used by GtkTreeViewColumn
to set the focus line width so that cells can paint a background on the
full background, then render themselves into the cell area... and parents
can go ahead and draw focus and other indicators on the background area
but outside of the cell area.
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.
