Notebook tab labels

[Owen Taylor <otaylor redhat com>]

While investigating some problems with warnings being
produced from the latest GTK+ and notebook, I
found some broken behavior with respect to tab
labels.

The way that tab labels have worked in the past is
that the notebook would automatically _show_
them when a tab label was added to a notebook.

However, in some circumstances, this automatic show
would not happen, and then the notebook would map
!VISIBLE tab labels. Which, with recent changes
produces nasty warnings.

For a container to automatically show its children
is simply not correct. However, I don't we can't
change this within the 1.2 branch. So, I'm planning
to fix this up as best as possible:

 - Change the notebook code so that it always
   shows tab labels added to it. (Not just sometimes)

 - Fix the case that was producing the warnings so
   that it doesn't try to map !VISIBLE widgets.
   (This is necessary, because an application could
   conceivably hide a tab label after adding it
   to a notebook)

But for 1.4, I think the automatic show needs to
go. Either this could be done as a simple documented
source-incompatible change, or it could be done
as part of a larger change that has been proposed - 
making all widgets default to VISIBLE.

Comments?
                                        Owen