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.
- 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
