Fix window button updates in split headers

Fix window button updates in split headers

We were only updating window buttons when the headerbar was
a direct child of the window. That is not the case in more
complicated situations, such as the split headers in gedit
or polari. To fix such cases, make the headerbar itself listen
for state changes on its toplevel.

https://bugzilla.gnome.org/show_bug.cgi?id=747805