treeview: Actually remove presize handler when we presize

Avoids multiple calls to it.
treeview: Reorder code

This just moves a function and is in preparation for the next commit.
treeview: Ensure we first validate the visible area when validating

This invariant stopped being guaranteed when we moved the visible area
validation from a high priority idle to a tick callback.

Fixes redrawing bugs like row expanding sometimes not having any visual
effect.
Don't use stock APIs in the widget factory
Don't use stock APIs in the bloatpad example
Remove usage of stock APIs in demo

With the exception of GtkAction.
GtkMenuTracker: fix leak when creating submenus
menu model: set separator label conditionally

When creating separators we were binding the "label" property on the
tracker to the "label" property on the GtkSeparatorMenuItem.

This was problematic for two reasons.

First, it was pointless.  The section header label will never change.

Second, it was causing problems: doing the binding caused the value to
be initially synced up, even if it was NULL.  Doing this caused
GtkMenuItem to create a GtkAccelLabel and add it as a child, which
prevented the separator from being shown normally.

Change the code a bit so that we just call gtk_menu_item_set_label()
when creating the item, if we find the label to be non-NULL.

Also, show() the separator item at first.  GtkMenu manages visibility of
separators internally, but it seems "more correct" to show it ourselves
at first.
Remove most of the stock API usage from the tests

With the exception of GtkActions and the tests for stock items.
Don't use stock APIs in the testsuite
Remove more discontinued ChangeLogs

They are all in git.
Start migrating internals away from GtkStock
gdkwindow: Don't recurse into clipped children when updating

We don't want to recurse into children that are clipped, as that is
wasted work. We handle this by moving the empty check to the top
of the function and only using the clipped region everywhere.
Document GtkTextSearchFlags

Move the doc about the flags from gtk_text_iter_forward_search() to the
enum.

https://bugzilla.gnome.org/show_bug.cgi?id=390048