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.
- 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