box gadget: Implement cross-axis alignment

So far, the box gadget is always allocating all children the
full size in the cross axis. This behavior corresponds to the
align-items: stretch behavior in
https://www.w3.org/TR/css-flexbox-1/#align-items-property
This commit implements the other modes described there.

While widgets have halign/valign properties that we can use for
this, the API for inserting gadgets has to change to take an
extra align parameter. All callers have been updated to pass
GTK_ALIGN_FILL, since that corresponds to the previous behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=760668

gtk/gtkboxgadgetprivate.h

@@ -21,6 +21,7 @@
 #define __GTK_BOX_GADGET_PRIVATE_H__
 
 #include "gtk/gtkcssgadgetprivate.h"
+#include "gtk/gtkenums.h"
 
 G_BEGIN_DECLS
 
@@ -65,10 +66,12 @@ void                    gtk_box_gadget_insert_gadget            (GtkBoxGadget
                                                                  int                     pos,
                                                                  GtkCssGadget           *cssgadget,
                                                                  gboolean                hexpand,
-                                                                 gboolean                vexpand);
+                                                                 gboolean                vexpand,
+                                                                 GtkAlign                align);
 void                    gtk_box_gadget_remove_gadget            (GtkBoxGadget           *gadget,
                                                                  GtkCssGadget           *cssgadget);
 
+
 G_END_DECLS
 
 #endif /* __GTK_BOX_GADGET_PRIVATE_H__ */