gtkheaderbar: update the window buttons on ::hierarchy-changed instead of ::realize

gtkheaderbar: update the window buttons on ::hierarchy-changed instead of ::realize

The window button setup depends on properties of the toplevel window.
Instead of updating the setup on realize, do it when the toplevel
changes.

This makes sure that when a GtkHeaderBar is added to a window
all the widgets are present and get_preferred_height() will return
the height the widget will have when finally shown. This allows
the logic in gtkwindow to select the right window size so that
the content size will match the requested default size.

https://bugzilla.gnome.org/show_bug.cgi?id=756618