Avoid a crash when calling _gtk_widget_get_first_child
Fixes https://github.com/gtk-rs/gtk4-rs/issues/889

Use inline getters for some things.

Replace leftover gtk-doc syntax (#Type) with backquotes.

This is now handle by the corresponding implemenatations using per
non-gobject-type API.

This makes it more explicit that managers of popovers make it "present".

We were setting the child-widget property to the parent.
Oops.

Since the LayoutManager owns the LayoutChild it creates, it's also
responsible for mopping them up.

Layout managers may need to get access to data attached to the root of a
scene graph.

Add a convenience api to skip children
that should not be included in the layout,
and call gtk_native_check_resize on all
native children outside of the vfunc.

We don't do that in GtkWidgetClass::measure() implementations either, so
make the semantics match.

Just like GtkContainer provides a default implementation of
GtkWidgetClass.get_request_mode(), we can do the same inside
GtkLayoutManager.

A default implementation preserves the behavior of existing widgets that
moved, or will move, to a GtkLayoutManager.

If we want to inspect the type of layout properties exposed by a
GtkLayoutManager, we need a way to connect the layout manager type to
the GtkLayoutChild type it creates. In order to do so, we can set the
GtkLayoutChild type on a field of the GtkLayoutManagerClass structure.

Storing the GtkLayoutChild type on the class structure of the layout
manager also allows us to implement a default create_layout_child()
virtual function.

The GtkWidget::parent-set signal was removed in ff6cd8f7.

Instead of removing GtkLayoutChild instances associated to a widget
using notifications when the widget's parent changes, we can have
gtk_widget_unparent() call a method on GtkLayoutManager to remove any
eventual GtkLayoutChild instances associated to the widget.

We can return a sensible default value, so we don't really need to warn
about a missing implementation.

 - Rename GtkLegacyLayout to GtkCustomLayout
 - Use for() to iterate over children in GtkBinLayout
 - Whitespace fixes for code imported from GtkBox
 - Store the GtkLayoutChild instances inside LayoutManager
 - Simplify the GtkLayoutManager API by dropping unnecessary arguments
 - Fix the ownership model of GtkLayoutManager

Layout managers needs a way to store properties that control the layout
policy of a widget; typically, we used to store these in GtkContainer's
child properties, but since GtkLayoutManager is decoupled from the
actual container widget, we need a separate storage. Additionally, child
properties have their own downsides, like requiring a separate, global
GParamSpecPool storage, and additional lookup API.

GtkLayoutChild is a simple GObject class, which means you can introspect
and document it as you would any other type.

We delegate the size request mode, the measuring, and the allocation of
a widget through a GtkLayoutManager instance, if one has been attached
to the widget; otherwise, we fall back to the widget's own implementation.

A base abstract class for layout manager delegate objects.

Layout managers are associated to a single widget, like event
controllers, and are responsible for measuring and allocating the
children of the widget they are bound to.